Revert 19736 because conflicts are avoided by clingo by default (#26721)

lib/spack/spack/build_systems/cuda.py

@@ -5,7 +5,6 @@
 
 import spack.variant
 from spack.directives import conflicts, depends_on, variant
-from spack.multimethod import when
 from spack.package import PackageBase
 
 
@@ -88,7 +87,7 @@ def cuda_flags(arch_list):
 
     # Linux x86_64 compiler conflicts from here:
     # https://gist.github.com/ax3l/9489132
-    with when('^cuda~allow-unsupported-compilers'):
+
     # GCC
     # According to
     # https://github.com/spack/spack/pull/25054#issuecomment-886531664

var/spack/repos/builtin/packages/cuda/package.py

@@ -122,8 +122,6 @@ class Cuda(Package):
     conflicts('arch=darwin-mojave-x86_64')
 
     variant('dev', default=False, description='Enable development dependencies, i.e to use cuda-gdb')
-    variant('allow-unsupported-compilers', default=False,
-            description='Allow unsupported host compiler and CUDA version combinations')
 
     depends_on('libxml2', when='@10.1.243:')
     # cuda-gdb needs libncurses.so.5