nfsd4: cl_count is unused

nfsd4: cl_count is unused

Now that the shutdown sequence guarantees callbacks are shut down before
the client is destroyed, we no longer have a use for cl_count.

We'll probably reinstate a reference count on the client some day, but
it will be held by users other than callbacks.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>