[RXRPC]: Remove Makefile reference to obsolete RXRPC config variable

Since there is no Kconfig variable RXRPC anywhere in the tree, and the
variable AF_RXRPC performs exactly the same function, remove the
reference to CONFIG_RXRPC from net/Makefile.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>