nfsd: remove recurring workqueue job to clean DRC

We have a shrinker, we clean out the cache when nfsd is shut down, and
prune the chains on each request. A recurring workqueue job seems like
unnecessary overhead. Just remove it.

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>