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/SampleUtils.cpp

@@ -143,7 +143,6 @@ nxt::TextureView CreateDefaultDepthStencilView(const nxt::Device& device) {
         .SetMipLevels(1)
         .SetAllowedUsage(nxt::TextureUsageBit::OutputAttachment)
         .GetResult();
-    depthStencilTexture.FreezeUsage(nxt::TextureUsageBit::OutputAttachment);
     return depthStencilTexture.CreateTextureViewBuilder()
         .GetResult();
 }