FreeBSD: VOP_MMAP has been dead since 1996

Clang complains that our (K&R!) declaration of struct vop_mmap_args
will only be visible within the (empty) function.
With the kernel's CFLAGS, though, this is fatal.
Remove the dead code.

Reviewed-on: http://gerrit.openafs.org/9856
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit a63e6747abd33fef40da16312dfc22d052bffd00)

Change-Id: I647af5c9574dacd2b9fcd5147a0a252b3981c2c0
Reviewed-on: http://gerrit.openafs.org/10376
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>