Remove waitingToShow of WindowToken

Except shell transition does not use the state, legacy transition
also doesn't need it because the condition mToken.isVisible()
already has the same meaning. The isVisible() will be true for a
visible-requested app when the transition is ready to commit the
visibility. And it is false if there is a transition which is not
ready yet.

Bug: 163976519
Test: CtsWindowManagerDeviceActivity
Change-Id: I649c3f673dfa2f8a9ef90f0f1b3fd21ff0030587