btrfs: send: remove unused code

This code was first introduced in 31db9f7c ("Btrfs: introduce
BTRFS_IOC_SEND for btrfs send/receive") and it was not functional, then
it got slightly refactored in e938c8ad

 ("Btrfs: code cleanups for
send/receive"), alas it was still dead. So let's remove it for good!

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