NFS: Remove all NFSIOS_FSCACHE counters due to conversion to netfs API

The old NFSIOS_FSCACHE counters are no longer accurate or useful with
the conversion to the new netfs API.  The new API does not have a page
based interface, and so the counters in nfs_stat_fscachecounters are
no longer obtainable.  The new netfs the API has extensive statistics
inside /proc/fs/fscache/stats so we no longer need NFS specific fscache
stats.

Note this also removes the 'fsc:' line from /proc/self/mountstats so
it will be a user-visible change.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Daire Byrne <daire@dneg.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>