Removing an Accessible removes descendants in the a11y tree.
However, there may be DOM descendants which have been relocated elsewhere in the a11y tree.
Their DOM nodes are now hidden as well, so we need to remove those Accessibles too.
In addition to Accessibles remaining in the tree when they shouldn't, failing to remove relocated Accessibles caused problems later on when a relocated Accessible was shown with new descendants.
Differential Revision: https://phabricator.services.mozilla.com/D41178
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