mon: remove check on pg_num for cephfs_pools

mon: remove check on pg_num for cephfs_pools

It should have been backported from 29a9dff but for better clarity I
think it's better to create a new commit for this.

c68126d6 aims to not make `pgs` attribute mandatory for each element of
`cephfs_pools`. Therefore, we must remove the check in
`roles/ceph-mon/tasks/check_mandatory_vars.yml`.
This task has been removed by 29a9dff but I've chosen to not backport
this commit since it's part of a bunch of commits belonging to a PR
implementing `ceph-validate` role.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>