btrfs: send: remove btrfs_debug() calls

There are debugging prints for each emitted send command and other
related actions. This does not seem right as the number of commands can
be high and dumping that to the system log will likely hit some rate
limiting.  This should be done by trace points that are more lightweight
and can keep up with high frequency.

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