UBI: do not select KALLSYMS_ALL

All UBI needs is to make sure we stacktraces when UBI debugging
is enabled. It is enough to select KALLSYMS for this, KALLSYMS_ALL
is not necessary.

And the current Kconfig line we have:

select KALLSYMS_ALL if KALLSYMS && DEBUG_KERNEL

is just too complex to be sane and right. But this "if" part there
is needed to prevent "unmet direct dependency" warnings, because
KALLSYMS_ALL depends on KALLSYMS and DEBUG_KERNEL, so we cannot
just select KALLSYMS_ALL.

Anyway, this feels messy, and we do not seem to really need KALLSYMS_ALL,
so select KALLSYMS instead.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>