rxrpc: remove unused static variables

The rxrpc_security_methods and rxrpc_security_sem user has been removed
in 648af7fc ("rxrpc: Absorb the rxkad security module"). This was
noticed by kbuild test robot for the -RT tree but is also true for !RT.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David Howells <dhowells@redhat.com>