Remove dup_select_related method.

This undocumented method was used in an old version of the admin, is
totally untested and hails from 2008. Although it's listed in the
"public methods" section, as it's not documented or used I don't think
it needs a deprecation path.

If we think it's useful I'll write some tests/docs for it instead...
Merge pull request #666 from mjtamlyn/dup-select-related

Remove dup_select_related method.