cmGlobalXCodeGenerator: Remove dead buildsystem version check

In commit 8d5f4c4db9 (Xcode: Switch to the "new build system" for Xcode
12 and above, 2020-09-14, v3.19.0-rc1~143^2~7) we accidentally added
code in an `else` block that under the opposite condition by which
the block can be entered.  Remove it.

Fixes: #22681

Source/cmGlobalXCodeGenerator.cxx

@@ -1756,9 +1756,6 @@ void cmGlobalXCodeGenerator::CreateCustomCommands(
       if (sourceFile->GetCustomCommand() &&
           visited.insert(sourceFile).second) {
         commands.push_back(*sourceFile->GetCustomCommand());
-        if (this->XcodeBuildSystem >= BuildSystem::Twelve) {
-          this->CustomCommandRoots[sourceFile].insert(gtgt);
-        }
       }
     }
     // create custom commands phase