Remove explicit usage transition from the API and validation

This removes the following for both Buffer and Texture:
 - The builder's SetInitialUsage
 - The object's FreezeUsage and TransitionUsage methods
 - The CommandBuffer Transition<Object>Usage methods

All samples and tests are simplified as a result. This also obsoletes
the UsageValidationTest which is removed.

Some validation was dependent on "current usage" and hasn't been
reintroduced for implicit transitions yet:
 - Buffers can be used while mapped
 - Swapchain textures can be used after they have been presented.

Validation for these will involve collecting all the resources used by a
command buffer and will be done in a follow-up patch.

examples/Animometer.cpp

@@ -156,7 +156,6 @@ void frame() {
     }
 
     queue.Submit(1, &commands);
-    backbuffer.TransitionUsage(nxt::TextureUsageBit::Present);
     swapchain.Present(backbuffer);
     DoFlush();
     fprintf(stderr, "frame %i\n", f);