clip: make remove_child a reverse of add_child

clip: make remove_child a reverse of add_child

Previously, we relied on ->child_removed to reverse the priority changes
that occured in ->add_child. However, ->child_removed is not always
called (the signal child-removed is not always emitted) when a
->add_child needs to be removed. However, ->remove_child is always
called to reverse ->add_child, so the code was moved here. Otherwise, we
risk that the priorities of the clip will contain gaps, which will cause
problems when another child is added to the clip.