BSD.usr.dist: remove empty /usr/libdata/gcc directory

I am unsure of its exact historical usage, but, we no longer ship GCC
with FreeBSD, and it should have been removed.

Reviewed by: imp, emaste
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44440

(cherry picked from commit cc1268a926772900a08c61fd791bb66df1e6c30c)