NFSD: Remove NFSD_TCP kernel build option

Likewise, distros usually leave CONFIG_NFSD_TCP enabled.

TCP support in the Linux NFS server is stable enough that we can leave it
on always.  CONFIG_NFSD_TCP adds about 10 lines of code, and defaults to
"Y" anyway.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>