Autogen: Remove <target_name>_autogen_timestamp_deps target

Since 2bfaaf6f6ed7481438bec08771992ea5152b5453 reorganized the dependency order for the `timestamp` target,
`<target_name>_autogen_timestamp_deps` is genetared as `phony edge with no inputs`. So this commit removes
`<target_name>_autogen_timestamp_deps`.

The dependency graph before
original dependencies of '_autogen' target <- _autogen_timestamp_deps <-'/timestamp' file <- '_autogen' target

The dependency graph after
original dependencies of '_autogen' target <-'/timestamp' file <- '_autogen' target

Fixes: #26080

Source/cmQtAutoGenInitializer.cxx

@@ -1480,6 +1480,16 @@ bool cmQtAutoGenInitializer::InitAutogenTarget()
       }
     }
 
+    // For the Ninja, Makefile and Qt >= 5.15, add custom commands that create
+    // XXX_autogen/timestamp files. Those custom commands have a depfile
+    // assigned that is generated from the depfiles that were created by moc.
+    //
+    // The XXX_autogen targets merely wrap the XXX_autogen/timestamp custom
+    // commands.
+    // The dependency tree would then look like
+    // the original dependencies of '_autogen' target <-'/timestamp' file
+    // <- '_autogen' target
+
     cmTarget* timestampTarget = nullptr;
     std::vector<std::string> dependencies(
       this->AutogenTarget.DependFiles.begin(),
@@ -1487,40 +1497,6 @@ bool cmQtAutoGenInitializer::InitAutogenTarget()
     if (useDepfile) {
       // Create a custom command that generates a timestamp file and
       // has a depfile assigned. The depfile is created by JobDepFilesMergeT.
-      //
-      // Also create an additional '_autogen_timestamp_deps' that the custom
-      // command will depend on. It will have no sources or commands to
-      // execute, but it will have dependencies that would originally be
-      // assigned to the pre-Qt 5.15 'autogen' target. These dependencies will
-      // serve as a list of order-only dependencies for the custom command,
-      // without forcing the custom command to re-execute.
-      //
-      // The dependency tree would then look like
-      // '_autogen_timestamp_deps (order-only)' <- '/timestamp' file <-
-      // '_autogen' target.
-      const auto timestampTargetName =
-        cmStrCat(this->GenTarget->GetName(), "_autogen_timestamp_deps");
-
-      auto cc = cm::make_unique<cmCustomCommand>();
-      cc->SetWorkingDirectory(this->Dir.Work.c_str());
-      cc->SetDepends(dependencies);
-      cc->SetEscapeOldStyle(false);
-      timestampTarget = this->LocalGen->AddUtilityCommand(timestampTargetName,
-                                                          true, std::move(cc));
-
-      this->LocalGen->AddGeneratorTarget(
-        cm::make_unique<cmGeneratorTarget>(timestampTarget, this->LocalGen));
-
-      // Set FOLDER property on the timestamp target, so it appears in the
-      // appropriate folder in an IDE or in the file api.
-      if (!this->TargetsFolder.empty()) {
-        timestampTarget->SetProperty("FOLDER", this->TargetsFolder);
-      }
-
-      // Make '/timestamp' file depend on '_autogen_timestamp_deps' and on the
-      // moc and uic executables (whichever are enabled).
-      dependencies.clear();
-      dependencies.push_back(timestampTargetName);
 
       AddAutogenExecutableToDependencies(this->Moc, dependencies);
       AddAutogenExecutableToDependencies(this->Uic, dependencies);
@@ -1565,7 +1541,7 @@ bool cmQtAutoGenInitializer::InitAutogenTarget()
           { cmSystemTools::GetCMakeCommand(), "-E", "touch", outputFile }));
         this->AddGeneratedSource(outputFile, this->Moc);
       }
-      cc = cm::make_unique<cmCustomCommand>();
+      auto cc = cm::make_unique<cmCustomCommand>();
       cc->SetOutputs(outputFile);
       cc->SetByproducts(timestampByproducts);
       cc->SetDepends(dependencies);