btrfs: remove btrfs_disk_cache_state

btrfs: remove btrfs_disk_cache_state

With neither the writer nor the loader left, nothing acts on
disk_cache_state. Remove it, the need_clear handling when reading block
groups, and the enum. While at it, drop the unused cache_generation
field from struct btrfs_block_group.

lookup_free_space_inode() converted old style space inodes by clearing
disk_cache_state so the cache would be rewritten with the new inode
flags. Without that it only sets flags on the in-memory inode, which
every remaining caller truncates or deletes right after, so drop the
conversion too.

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