Moves the `Drop` implementation for `GPUPipelineLayout` to a dedicated
private helper struct, `DroppableGPUPipelineLayout`. This ensures that
DOM types do not directly implement `Drop`, aligning with best practices
for resource management in the DOM. The associated `Bindings.conf` entry
is removed as direct `Drop` is no longer needed for this type.
Testing: WebGpu just coverages cases with proper tests
Fixes: Partially #26488
Signed-off-by: Domenico Rizzo <domenico.rizzo@gmail.com>