CMake: Remove "Prefer Ninja" checkbox on CMake Tool page

This is no longer needed now that the generator is a property of the
Kit.

Change-Id: Ife35fd9207c805a4ead1e067603df1bfc77e6855
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/plugins/cmakeprojectmanager/cmakeprojectmanager.cpp

@@ -126,11 +126,6 @@ QString CMakeManager::mimeType() const
     return QLatin1String(Constants::CMAKEPROJECTMIMETYPE);
 }
 
-bool CMakeManager::preferNinja()
-{
-    return CMakeToolManager::preferNinja();
-}
-
 // need to refactor this out
 // we probably want the process instead of this function
 // cmakeproject then could even run the cmake process in the background, adding the files afterwards