bcachefs: Kill allocator short-circuit invalidate

bcachefs: Kill allocator short-circuit invalidate

The allocator thread invalidates buckets (increments their generation
number) prior to discarding them and putting them on freelists. We've
had a short circuit path for some time to only update the in-memory
bucket mark when doing the invalidate if we're not invalidating cached
data, but that short-circuit path hasn't really been needed for quite
some time (likely since the btree key cache code was added).

We're deleting it now as part of deleting/converting code that uses the
in memory bucket array.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>