btrfs-progs: Remove unneeded assert in find_good_parent().

btrfs-progs: Remove unneeded assert in find_good_parent().

find_good_parent() uses assert to deal with the problem that clone
source's parent can't be found.
But in fact the assert is somewhat overkilled since subvol_uuid_search()
has enough error messages for debug and caller of find_good_parent() can
handle the problems in find_good_parent(), so the assert can be removed
without any problem.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>