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.

Change-Id: I8fd768b36b73f9e2f727dae4e748931f307444c8
Reviewed-on: http://gerrit.openafs.org/9856
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>