northbridge/intel/nehalem/northbridge.c: Remove unused variable

Spotted by Clang.

Change-Id: I17e64ee989b611fac91072b9e97eab168cfae525
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6128
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>