Consider removing GPUPipelineLayout #2550

The motivation for this is:

• Partially implicit pipeline layouts: Consider removing or reworking implicit pipeline layouts #2470 (comment)
• Pipeline layouts are entirely represented by their value.
• The driver probably doesn't do much work inside the creation of a pipeline layout objects.
• WebGPU doesn't actually take advantage of pipeline layout objects. There's no pipeline layout object in setBindGroup (like in vkCmdBindDescriptorSets).

Editors weren't 100% sure if we still might want pipeline layout objects in the future (for things like push constants, immutable samplers, pipeline layout compatibility?), though they could always be added back.

Sorry, going on vacation now. More details later :P