kassert: Remove KASAN marking from DEBUG_POISON_POINTER

While this approach works for trapping reads of an uninitialized
pointer, it means that any attempt to store to the variable triggers a
KASAN report, which is not what we want. Simply remove the kasan_mark()
call. KMSAN will catch these kinds of bugs automatically anyway.

Reported by: syzbot+c5e279db5763b58e8d69@syzkaller.appspotmail.com
MFC after: 1 week