Ninja: Use clean target instead of clean tool in cmake --target clean calls

A convenience `clean` target for the Ninja generator exists since commit
3bd41f2eb5.  It's safe to call `ninja clean` instead of `ninja -t clean`.
This removes the exception mapping of the `clean` target in
`cmake --build ... --target clean` calls to the Ninja `-t clean` tool.

Source/cmGlobalNinjaGenerator.cxx

@@ -679,11 +679,7 @@ cmGlobalNinjaGenerator::GenerateBuildCommand(
   makeCommand.Add(makeOptions.begin(), makeOptions.end());
   for (const auto& tname : targetNames) {
     if (!tname.empty()) {
-      if (tname == "clean") {
-        makeCommand.Add("-t", "clean");
-      } else {
-        makeCommand.Add(tname);
-      }
+      makeCommand.Add(tname);
     }
   }
   return { std::move(makeCommand) };

Source/cmGlobalNinjaGenerator.h

@@ -43,8 +43,6 @@ struct cmDocumentationEntry;
  *   it is handle by Ninja's -v option.
  * - We don't care about computing any progress status since Ninja manages
  *   it itself.
- * - We don't care about generating a clean target since Ninja already have
- *   a clean tool.
  * - We generate one build.ninja and one rules.ninja per project.
  * - We try to minimize the number of generated rules: one per target and
  *   language.