GrampsLocale: Don't set  to 'C' if LANG is en_US

Not required, Gtk has an 'en' translation

Ref. http://www.gramps-project.org/bugs/view.php?id=6867

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@22710 4ae1f11a-8b86-4847-b8af-ab372f36d1fd