DEV: Drop plugin related index from core (#20930)

The migration of the index has since been moved into the
discourse-akismet plugin where it should have always been.

db/migrate/20230403063113_drop_idx_post_custom_fields_akismet.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+# This plugin related index was added to core way back in 2018 but it should not have been added to core in the first place.
+# The index has since been moved into the plugin itself.
+class DropIdxPostCustomFieldsAkismet < ActiveRecord::Migration[7.0]
+  def up
+    execute "DROP INDEX IF EXISTS idx_post_custom_fields_akismet"
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end