btrfs: remove the v1 space cache writeout from the transaction commit

btrfs: remove the v1 space cache writeout from the transaction commit

Nothing sets SPACE_CACHE anymore, so the dirty block group writers never
have a cache to write out or wait for. Remove cache_save_setup(),
btrfs_setup_space_cache(), the io_list handling, the io_bgs list and
BTRFS_TRANS_CACHE_ENOSPC, and the abort-time cleanup of in-flight cache
IO.

The -ENOENT retry in btrfs_write_dirty_block_groups() handled a free
space endio worker creating a block group during the commit critical
section, so drop it too.

Assisted-by: Claude:claude-fable-5-1
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Signed-off-by: David Sterba <dsterba@suse.com>