bsd.port.mk.5: GC the misp64 *pic.a fragment handling documentation

bsd.port.mk.5: GC the misp64 *pic.a fragment handling documentation

More than ten years ago, bsd.lib.mk stopped creating lib*_pic.a files
and the corresponding PFRAG.no_mips64 were removed from the ports tree.
Last year the -Dno_mips64 handling in bsd.port.mk was GC'd too, but the
note in the bsd.port.mk manpage was left.

ok espie@