btrfs: remove the v1 space cache load path

btrfs: remove the v1 space cache load path

Nothing writes a v1 space cache any more, and since commit 545e560a5b0f
("btrfs: disable v1 space cache") the mount option can't be enabled to
read one either. Remove load_free_space_cache(), its io_ctl helpers and
struct btrfs_io_ctl. Drop the gfp constraint on the inode mapping as
well, it only covered the cache's page cache allocations.

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