[engine] Remove the unused Transaction online_id property

The "online-id" GObject property on Transaction (KVP slot "online_id")
was vestigial: GnuCash records the OFX/HBCI import identifier on the
bank/asset Split, not on the Transaction.  Drop the property, its
get/set_property cases, the xaccTransClone special-case that cleared it,
and the corresponding test_trans_kvp_properties coverage.

libgnucash/engine/Transaction.cpp

@@ -200,7 +200,6 @@ enum
     PROP_DESCRIPTION,	/* Table */
     PROP_INVOICE,	/* KVP */
     PROP_SX_TXN,	/* KVP */
-    PROP_ONLINE_ACCOUNT,/* KVP */
 };
 
 void
@@ -331,9 +330,6 @@ gnc_transaction_get_property(GObject* object,
     case PROP_SX_TXN:
         qof_instance_get_kvp (QOF_INSTANCE (tx), value, 1, GNC_SX_FROM);
         break;
-    case PROP_ONLINE_ACCOUNT:
-        qof_instance_get_kvp (QOF_INSTANCE (tx), value, 1, "online_id");
-        break;
     default:
         G_OBJECT_WARN_INVALID_PROPERTY_ID(object, prop_id, pspec);
         break;
@@ -379,9 +375,6 @@ gnc_transaction_set_property(GObject* object,
     case PROP_SX_TXN:
         qof_instance_set_kvp (QOF_INSTANCE (tx), value, 1, GNC_SX_FROM);
         break;
-    case PROP_ONLINE_ACCOUNT:
-        qof_instance_set_kvp (QOF_INSTANCE (tx), value, 1, "online_id");
-        break;
     default:
         G_OBJECT_WARN_INVALID_PROPERTY_ID(object, prop_id, pspec);
         break;
@@ -470,16 +463,6 @@ gnc_transaction_class_init(TransactionClass* klass)
 			   "transactions",
 			   GNC_TYPE_GUID,
 			   G_PARAM_READWRITE));
-
-    g_object_class_install_property
-    (gobject_class,
-     PROP_ONLINE_ACCOUNT,
-     g_param_spec_string ("online-id",
-                          "Online Account ID",
-                          "The online account which corresponds to this "
-			  "account for OFX/HCBI import",
-                          nullptr,
-                          G_PARAM_READWRITE));
 }
 
 /********************************************************************\
@@ -678,9 +661,6 @@ xaccTransClone (const Transaction *from)
     xaccTransBeginEdit (to);
     qof_instance_copy_kvp (QOF_INSTANCE (to), QOF_INSTANCE (from));
 
-    /* But not the online-id! */
-    qof_instance_set (QOF_INSTANCE (to), "online-id", nullptr, nullptr);
-
     for (GList* lfrom = from->splits, *lto = to->splits; lfrom && lto;
          lfrom = g_list_next (lfrom), lto = g_list_next (lto))
         xaccSplitCopyKvp (GNC_SPLIT(lfrom->data), GNC_SPLIT(lto->data));

libgnucash/engine/Transaction.h

@@ -352,7 +352,6 @@ const char *  xaccTransGetDocLink(const Transaction *trans);
  The Notes field is only visible in the register in double-line mode */
 const char *  xaccTransGetNotes (const Transaction *trans);
 
-
 /** Sets whether or not this transaction is a "closing transaction" */
 void          xaccTransSetIsClosingTxn (Transaction *trans, gboolean is_closing);
 

libgnucash/engine/test/test-engine-kvp-properties.c

@@ -189,14 +189,11 @@ test_trans_kvp_properties (Fixture *fixture, gconstpointer pData)
     GncGUID *invoice = guid_new ();
     GncGUID *from_sx = guid_new ();
     GncGUID *invoice_r, *from_sx_r;
-    gchar *online_id = "my online id";
-    gchar *online_id_r;
 
     xaccTransBeginEdit (fixture->trans);
     qof_instance_set (QOF_INSTANCE (fixture->trans),
 		      "invoice", invoice,
 		      "from-sched-xaction", from_sx,
-		      "online-id", online_id,
 		      NULL);
 
     g_assert_true (qof_instance_is_dirty (QOF_INSTANCE (fixture->trans)));
@@ -205,17 +202,14 @@ test_trans_kvp_properties (Fixture *fixture, gconstpointer pData)
     qof_instance_get (QOF_INSTANCE (fixture->trans),
 		      "invoice", &invoice_r,
 		      "from-sched-xaction", &from_sx_r,
-		      "online-id", &online_id_r,
 		      NULL);
     g_assert_true (guid_equal (invoice, invoice_r));
     g_assert_true (guid_equal (from_sx, from_sx_r));
-    g_assert_cmpstr (online_id, ==, online_id_r);
     g_assert_true (!qof_instance_is_dirty (QOF_INSTANCE (fixture->trans)));
     guid_free (invoice);
     guid_free (invoice_r);
     guid_free (from_sx);
     guid_free (from_sx_r);
-    g_free (online_id_r);
 }
 
 static void