btrfs: Clean up dead code in root-tree

The value of variable 'can_recover' is never used after being set, thus
it should be removed, as it was never used since the first commit
68a7342c ("Btrfs: cleanup orphaned root orphan item").

Signed-off-by: Christos Gkekas <chris.gekas@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>