btrfs: remove the v1 space cache write path

btrfs: remove the v1 space cache write path

Nothing writes out a v1 space cache any more. Remove the writers and
their io_ctl helpers, along with create_free_space_inode() and
btrfs_prealloc_file_range_trans(), whose only user was the cache inode
creation. The io_list and io_ctl block group fields were only used by
the writers, so remove them too.

btrfs_truncate_free_space_cache() only needed the block group to wait
for and clear in-flight cache IO, so drop that parameter.

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