afs: cell: Remove unnecessary code in afs_lookup_cell

Due to recent changes this piece of code is no longer needed.

Addresses-Coverity-ID: 1462033
Link: https://lkml.kernel.org/r/4923.1510957307@warthog.procyon.org.uk


Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: David Howells <dhowells@redhat.com>