bcachefs: journal seq blacklist gc no longer has to walk btree

Since btree_ptr_v2, we no longer require the journal seq blacklist table
for skipping blacklisted bsets (btree node entries); the pointer to a
given node indicates how much data is present.

Therefore there's no longer any need for journal seq blacklist gc to
walk the btree - we can prune entries older than journal last_seq.

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