kernel: unconditionally disable /dev/kmem

This was presumably set for grsecurity compatibility, but now appears
redundant.  Grsecurity does not expect nor require /dev/kmem to be
present and so it makes little sense to continue making its inclusion in
the standard kernel dependent on grsecurity.

More generally, given the large number of possible grsecurity
configurations, it is unclear what `features.grsecurity` even
*means* and its use should be discouraged.