cpu-common.h: Remove a pointless ifndef CONFIG_USER_ONLY

Remove an ifndef CONFIG_USER_ONLY guard that was pointless
because it is already inside an if !defined(CONFIG_USER_ONLY).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
