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, 0 insertions, 25 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index 2a641cbe53..ec8af73d07 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -368,31 +368,6 @@ module ActiveRecord
           SQL
         end
 
-        def bulk_change_table(table_name, operations)
-          sql_fragments = []
-          non_combinable_operations = []
-
-          operations.each do |command, args|
-            table, arguments = args.shift, args
-            method = :"#{command}_for_alter"
-
-            if respond_to?(method, true)
-              sqls, procs = Array(send(method, table, *arguments)).partition { |v| v.is_a?(String) }
-              sql_fragments << sqls
-              non_combinable_operations.concat(procs)
-            else
-              execute "ALTER TABLE #{quote_table_name(table_name)} #{sql_fragments.join(", ")}" unless sql_fragments.empty?
-              non_combinable_operations.each(&:call)
-              sql_fragments = []
-              non_combinable_operations = []
-              send(command, table, *arguments)
-            end
-          end
-
-          execute "ALTER TABLE #{quote_table_name(table_name)} #{sql_fragments.join(", ")}" unless sql_fragments.empty?
-          non_combinable_operations.each(&:call)
-        end
-
         # Renames a table.
         # Also renames a table's primary key sequence if the sequence name exists and
         # matches the Active Record default.