Drop the index only for postgresql, because

mysql cannot simply drop the index without dropping the corresponding
foreign key, and we certainly don't want to drop the foreign key
here.

db/migrate/20170130204620_add_index_to_project_authorizations.rb

@@ -10,6 +10,7 @@ class AddIndexToProjectAuthorizations < ActiveRecord::Migration
   end
 
   def down
-    remove_index(:project_authorizations, :project_id)
+    remove_index(:project_authorizations, :project_id) if
+      Gitlab::Database.postgresql?
   end
 end