[a11y] Delete old a11y code.

1. Delete old semantic tree code since this has been replaced
by new code under a11y_manager/semantics/.
2. Delete A11y_toggler code.
3. Fuchsiavox will no longer be built. I am not deleting the code
   for reference pursposes and will be deleted once the new implemenation
   is in place.

Testing:
  * Build: passed
  * Installed on device
  * Unit Test: Passed
      Command: fx run-test a11y_tests

MI4-1736 #comment

Change-Id: I4b558190ce144e50bd7fba78c882aeaeba60f953