Revert "btrfs-progs: check: supplement extent backref list with rbtree"

Revert "btrfs-progs: check: supplement extent backref list with rbtree"

This reverts commit 31d8235410985e0b64487354c9ba67d40c4bdfe3.

False report of backref mismatches, lots of messages similar to:

Incorrect local backref count on 12713984 root 5 owner 257 offset 12845056 found 1 wanted 0 back 0x7b3ed0
backpointer mismatch on [12713984 131072]

Repairing will make things worse. A fix has been proposed, but is not
finalized so we go with a revert.

Reported-by: Chris Murphy <bugzilla@colorremedies.com>
References: https://bugzilla.kernel.org/show_bug.cgi?id=155791
Signed-off-by: David Sterba <dsterba@suse.com>