Remove usr/lib/libssp.a.

GNU's libssp installed this (in addition to libssp_nonshared.a), but
the libc-based libssp does not.

Reviewed by: kevans, emaste
Fixes: cd0d51baaa45 Provide libssp based on libc
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D33852