Remove unused quoted_locking_column method.

This method isn't used anywhere.

This added at https://github.com/rails/rails/commit/722e0b6a8bb62226437e466b5a98356d026b05f5 in 2006.
Maybe, when we were starting to use arel, we forgot to remove this.

If this is public api, we must not remove this, and should deprecate this.