Schema#removeDisallowedAttributes does not check all children of given element

[oskarwrobel]

When an element with children is passed to the Schema#removeDisallowedAttributes it may happen that some elements won't be cleared.

<parent>
  <$text foo="a">a</$text><$text foo="b">b</$text><$text foo="c">c</$text>
</parent>

When all attributes are disallowed in text for the parent element, then the last one won't be cleared.

<parent>
  ab<$text foo="c">c</$text>
</parent>

While removing attributes nodes are merged and this affects iterator.

[Reinmar]

Closed with ckeditor/ckeditor5-engine#1790.