pcmcia: cs: Remove unused pcmcia_get_socket_by_nr

The last use of pcmcia_get_socket_by_nr() was removed in 2010 by
commit 5716d415 ("pcmcia: remove obsolete ioctl")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>