use PostgreSQL's bulk_alter_table implementation

Running this migration on mysql at current master fails
because `add_references_for_alter` is missing.

```
change_table :users, bulk: true do |t|
  t.references :article
end
```

This is also true for postgresql adapter,
but its `bulk_alter_table` implementation can fallback in such case.
postgresql's implementation is desirable to prevent unknown failure like this.

1 files changed, 1 insertions, 16 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index ca8bbc14da..2a2b234ef8 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -63,7 +63,7 @@ module ActiveRecord
         /mariadb/i.match?(full_version)
       end
 
-      def supports_bulk_alter? #:nodoc:
+      def supports_bulk_alter?
         true
       end
 
@@ -285,21 +285,6 @@ module ActiveRecord
         SQL
       end
 
-      def bulk_change_table(table_name, operations) #:nodoc:
-        sqls = operations.flat_map do |command, args|
-          table, arguments = args.shift, args
-          method = :"#{command}_for_alter"
-
-          if respond_to?(method, true)
-            send(method, table, *arguments)
-          else
-            raise "Unknown method called : #{method}(#{arguments.inspect})"
-          end
-        end.join(", ")
-
-        execute("ALTER TABLE #{quote_table_name(table_name)} #{sqls}")
-      end
-
       def change_table_comment(table_name, comment) #:nodoc:
         comment = "" if comment.nil?
         execute("ALTER TABLE #{quote_table_name(table_name)} COMMENT #{quote(comment)}")