btrfs: Remove unused extent_root var from caching_thread

Added by b4570aa9 ("btrfs: fix compiling with CONFIG_BTRFS_DEBUG
enabled.") and obsoleted by 2ff7e61e

 ("btrfs: take an fs_info
directly when the root is not used otherwise").

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>