Minor cleanup: remove unnecessary GetCudaContext.

Note that there is no protection against a caller of CUDAExecutor::Launch from
accidentally passing a Stream associated with the wrong CUDAExecutor.

This is no different from any other CUDAExecutor methods that take a Stream
argument, where we similarly have no such protection.

The main caller is Stream::ThenLaunch, which necessarily calls Launch on the
correct corresponding CUDAExecutor. Other callers use a similar pattern.

PiperOrigin-RevId: 176719918