Revert "Use virtual architecture to setup default CUDA_ARCH"

This reverts commit 8b7d0d05.
An unintended consequence is that now only PTX is generated by
default, unless CUDA_NVCC_FLAGS is used to generate code for
specific devices.

CMakeLists.txt

@@ -133,7 +133,7 @@ if(CUDA)
   endif()
 
   set(CUDA_ARCH 30 CACHE STRING "CUDA device architecture.")
-  set(CUDA_ARCH "-arch=compute_${CUDA_ARCH}")
+  set(CUDA_ARCH "-arch=sm_${CUDA_ARCH}")
 
   set(VecCore_COMPONENTS ${VecCore_COMPONENTS} "CUDA")
 endif()