btrfs: remove unused btrfs_root::type

Looks like the struct member was added in 2007 in 2.6.29 in commit
87ee04eb ("Btrfs: Add simple stripe size parameter") but hasn't been
used at all since. So let's remove it. This was found by tool
https://github.com/jirislaby/clang-struct, then build tested after
removing the struct member.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -212,8 +212,6 @@ struct btrfs_root {
 
 	u64 last_trans;
 
-	u32 type;
-
 	u64 free_objectid;
 
 	struct btrfs_key defrag_progress;