s390/traps: Get rid of superfluous cpu_has_vx() check

If the vector facility is installed cpu_has_vx() is always true, if it is
not installed the result is always false, and no vector exception can
happen. Therefore remove the superfluous check.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>