xtensa: remove obsolete BKL kernel option from defconfig

The BKL doesn't exist anymore so remove CONFIG_LOCK_KERNEL.

This work was supported by a hardware donation from the CE Linux Forum.

Signed-off-by: Alessio Igor Bogani <abogani@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>