bcachefs: delete faulty fastpath in bch2_btree_path_traverse_cached()

bch2_btree_path_traverse_cached() was previously checking if it could
just relock the path, which is a common idiom in path traversal.

However, it was using btree_node_relock(), not btree_path_relock();
btree_path_relock() only succeeds if the path was in state
BTREE_ITER_NEED_RELOCK.

If the path was in state BTREE_ITER_NEED_TRAVERSE a full traversal is
needed; this led to a null ptr deref in
bch2_btree_path_traverse_cached().

And the short circuit check here isn't needed, since it was already done
in the main bch2_btree_path_traverse_one().

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