Update configuring.md [ci skip]

Remove ActiveRecord config instructions for active_record.attribute_types_cached_by_default, which was removed in https://github.com/rails/rails/commit/66736c8e50585
Merge pull request #16597 from teeparham/master

Remove cached attributes from Configuring guide [ci skip]

This was removed in #15429
Fix SyntaxError
"warning: assigned but unused variable"