libbtrfs: remove the support for fs without uuid tree

Since kernel 3.12, any btrfs mounted by a kernel would have an UUID
tree created, to record all the UUID of its subvolumes.

Without UUID tree, libbtrfs send functionality has to go through all the
subvolumes seen so far, and record those subvolumes' UUID internally so
that libbtrfs send can locate a desired subvolume.

Since commit 194b90a ("btrfs-progs: libbtrfs: remove declarations
without exports in send-utils") we're deprecating this old interface
already, meaning deprecated users won't be able to build its own
subvolume list already.

And we received no error report on this so far.  So let's finish the
cleanup by removing the support for fs without an UUID tree.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710