Removed unneeded conflicts in CudaPackage and RocmPackage (#29699)

Since the variant is conditional there's no need to add
conflicts too.

lib/spack/spack/build_systems/cuda.py

@@ -188,7 +188,3 @@ def cuda_flags(arch_list):
         # Darwin.
         # TODO: add missing conflicts for %apple-clang cuda@:10
         conflicts('platform=darwin', when='+cuda ^cuda@11.0.2: ')
-
-    # Make sure cuda_arch can not be used without +cuda
-    for value in cuda_arch_values:
-        conflicts('~cuda', when='cuda_arch=' + value)

lib/spack/spack/build_systems/rocm.py

@@ -112,10 +112,6 @@ class ROCmPackage(PackageBase):
     # need amd gpu type for rocm builds
     conflicts('amdgpu_target=none', when='+rocm')
 
-    # Make sure amdgpu_targets cannot be used without +rocm
-    for value in amdgpu_targets:
-        conflicts('~rocm', when='amdgpu_target=' + value)
-
     # https://github.com/ROCm-Developer-Tools/HIP/blob/master/bin/hipcc
     # It seems that hip-clang does not (yet?) accept this flag, in which case
     # we will still need to set the HCC_AMDGPU_TARGET environment flag in the