Remove the old NFS lock device driver that uses Giant.

This NFS lock device driver was replaced by the kernel NLM around FreeBSD7 and
has not normally been used since then.
To use it, the kernel had to be built without "options NFSLOCKD" and
the nfslockd.ko had to be deleted as well.
Since it uses Giant and is no longer used, this patch removes it.

With this device driver removed, there is now a lot of unused code
in the userland rpc.lockd. That will be removed on a future commit.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D22933