share/mk: Remove more support for building profiled libraries

• Remove documentation of profiled library support from bsd.README

• Remove rules to create individual .po files. The rules to generate an archive from these files was already removed, so these .po rules aren't used.

Reviewed by: emaste
Fixes: 3750ccefb862 ("Retire MK_PROFILE infrastructure")
Differential Revision: https://reviews.freebsd.org/D48819