bcachefs: Delete dead code from bch2_discard_one_bucket()

bcachefs: Delete dead code from bch2_discard_one_bucket()

alloc key validation ensures that if a bucket is in need_discard state
the sector counts are all zero - we don't have to check for that.

The NEED_INC_GEN check appears to be dead code, as well: we only see
buckets in the need_discard btree, and it's an error if they aren't in
the need_discard state.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>