Moves the `Drop` implementation for `GPUDevice` into a private
`DroppableGPUDevice` struct.
Also updates `Bindings.conf` to prevent the generation of `Drop`
implementations for `GPUDevice`.
Testing: WebGpu tests just cover the cases
Fixes: Partially #26488
Signed-off-by: Domenico Rizzo <domenico.rizzo@gmail.com>