Remove WindowContextCleaner

Since Context#destroy calls to ContextImpl, which holds reference of
WindowContext as outerContext, the WindowContext won't be finalized,
which led to WindowContextCleaner doesn't work.

The finalize timeout issue should bot happen any more since we update
WMS#detachWindowContext to one-way.

Flag: EXEMPT remove flag
Test: build
Fixes: 339727951
Change-Id: If3b1212e7d78efe9c65889d14778afacf305269d