afs: remove variable nr_servers

Variable nr_servers is no longer being used, the last reference
to it was removed in commit 45df8462

 ("afs: Fix server list handling")
so clean up the code by removing it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/20221020173923.21342-1-colin.i.king@gmail.com/