s390: replace defconfig with performance_defconfig

Replace defconfig with performance_defconfig. defconfig had some more
or less random debug options enabled, where nobody knows why anymore.

Just remove the old defconfig and replace it with performance_defconfig,
which reduces the number of configs to maintain. A config with debugging
options enabled is debug_defconfig which is supposed to be rather close
to performance_defconfig except that is has debug options enabled.
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>