Bug 1821963 part 3: Remove various components supporting the old accessibility architecture based on content process COM proxies. r=nlapre,nalexander

This includes AccessibleHandler, HandlerProvider, IGeckoCustom and the IAccessible2 COM proxy dll.
Even with the new architecture, we still use IAccessible2, but we no longer need a COM proxy because we aren't using COM across processes ourselves.
If clients want to use IAccessible2 across processes, they're responsible for registering a COM proxy themselves as with all other IAccessible2 applications.
Alternatively, they can rely on the IAccessible2 COM proxy which is included with Windows 10 and later.

Differential Revision: https://phabricator.services.mozilla.com/D177963