Remove the description of MAP_HASSEMAPHORE.

The flag is not implemented, all FreeBSD architectures correctly
handle locks on normal cacheable mappings. On the other hand, the
flag was specified by some software, so it is kept in the header as
nop. Removal from the man page should discourage its use.

Reviewed by: alc, bjk, emaste, markj, pho
MFC after: 3 days
X-Differential revision: https://reviews.freebsd.org/D11306