Sparc: sparc_cpu_model isn't in asm/system.h any more [ver #2]

sparc_cpu_model isn't in asm/system.h any more, so remove a comment
about it.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: sparclinux@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>