Convert fc-cache(1) calls in pkg-plist to use @fc keyword

fonts.cache-1 lines started to appear after r148143 but became
obsolete since fc-cache -s support (i.e. /var/db/fontconfig)
and several FC_CACHE_VERSION bumps. Remove them.

Differential Revision: https://reviews.freebsd.org/D1733
Reviewed by: hrs
Approved by: portmgr blanket
Approved by: bapt (mentor)