[Ada] CUDA: remove code performing kernel registration

[Ada] CUDA: remove code performing kernel registration

A previous commit implemented a new kernel registration scheme, using
the binder to generate registration code rather than inserting
registration code in packages.  Now that this new approach has had time
to be thoroughly tested, it is time to remove the old approach.

gcc/ada/

	* gnat_cuda.ads: Update package-level comments.
	(Build_And_Insert_CUDA_Initialization): Remove function.
	* gnat_cuda.adb (Build_And_Insert_CUDA_Initialization): Remove
	function.
	(Expand_CUDA_Package): Remove call to
	Build_And_Insert_CUDA_Initialization.