UBIFS: kill CONFIG_UBIFS_FS_DEBUG_CHKS

Simplify UBIFS configuration menu and kill the option to enable self-check
compile-time. We do not really need this because we can do this run-time
using the module parameters or the corresponding sysfs interfaces. And
there is a value in simplifying the kernel configuration menu which becomes
increasingly large.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>