cephfs: remove useMount test helper function
cephfs: remove useMount test helper function

Now that there are no tests using the fuse mount point and the useMount
function that detects an external mount point is needed, we can remove
the useMount function itself and the env var used to control it.

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph