DEV: Drop duration column from topic timers (#13543)

The duration column has been ignored since the commit
4af77f1e38
for topic_timers, we use duration_minutes instead.

Also removing the duration key from Topic.set_or_create_timer. The only
plugin to use this was discourse-solved, which doesn't use it any
longer
since
c722b94a97

db/post_migrate/20210628035905_drop_duration_column_from_topic_timers.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class DropDurationColumnFromTopicTimers < ActiveRecord::Migration[6.1]
+  DROPPED_COLUMNS ||= {
+    topic_timers: %i{duration}
+  }
+
+  def up
+    DROPPED_COLUMNS.each do |table, columns|
+      Migration::ColumnDropper.execute_drop(table, columns)
+    end
+  end
+
+  def down
+    add_column :topic_timers, :duration, :string
+  end
+end