Moves the `Drop` logic for `GPURenderPipeline` into a dedicated helper
struct, `DroppableGPURenderPipeline`.
The associated `Bindings.conf` entry is
removed as direct `Drop` is no longer needed for this type.
Testing: WebGpu tests just cover its cases
Fixes: Partially #26488
Signed-off-by: Domenico Rizzo <domenico.rizzo@gmail.com>