NFSD: Remove the nfsd_cb_work and nfsd_cb_done tracepoints

Clean up: These are noise in properly working systems. If you really
need to observe the operation of the callback mechanism, use the
sunrpc:rpc\* tracepoints along with the workqueue tracepoints.

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