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>