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.

app/models/post_custom_field.rb

@@ -17,7 +17,6 @@ end
 #
 # Indexes
 #
-#  idx_post_custom_fields_akismet                (post_id) WHERE (((name)::text = 'AKISMET_STATE'::text) AND (value = 'needs_review'::text))
 #  index_post_custom_fields_on_name_and_value    (name, "left"(value, 200))
 #  index_post_custom_fields_on_notice            (post_id) UNIQUE WHERE ((name)::text = 'notice'::text)
 #  index_post_custom_fields_on_post_id           (post_id) UNIQUE WHERE ((name)::text = 'missing uploads'::text)

db/migrate/20180327062911_add_post_custom_fields_akismet_index.rb

@@ -1,15 +0,0 @@
-# frozen_string_literal: true
-
-# This is our current pattern for data migrations needed by plugins, we prefer to keep them in core
-# so schema is tightly controlled, especially if we are amending tables owned by core
-#
-# this index makes looking up posts requiring review much faster (20ms on meta)
-
-class AddPostCustomFieldsAkismetIndex < ActiveRecord::Migration[5.1]
-  def change
-    add_index :post_custom_fields,
-              [:post_id],
-              name: "idx_post_custom_fields_akismet",
-              where: "name = 'AKISMET_STATE' AND value = 'needs_review'"
-  end
-end

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