Revert "upcxx %oneapi@2025: cxxflags add -Wno-error=missing-template-arg-list-after-template-kw (#47503)" (#47512)

This reverts commit 4322cf56b1.

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

@@ -164,11 +164,7 @@ class Upcxx(Package, CudaPackage, ROCmPackage):
     depends_on("oneapi-level-zero@1.8.0:", when="+level_zero")
 
     # All flags should be passed to the build-env in autoconf-like vars
-    def flag_handler(self, name, flags):
-        if name == "cxxflags":
-            if self.spec.satisfies("%oneapi@2025:"):
-                flags.append("-Wno-error=missing-template-arg-list-after-template-kw")
-        return (flags, None, None)
+    flag_handler = env_flags
 
     def set_variables(self, env):
         env.set("UPCXX_INSTALL", self.prefix)