Remove redundant CMAKE_GENERATOR parameter from CTK project

The 'CMAKE_GENERATOR' property is already added by calling
'superbuild_include_dependencies' and associating
'${CTK_EXTERNAL_PROJECT_ARGS}' with the project.

See #418

SuperBuild.cmake

@@ -37,7 +37,6 @@ set(proj CTK)
 ExternalProject_Add(${proj}
   ${CTK_EXTERNAL_PROJECT_ARGS}
   DOWNLOAD_COMMAND ""
-  CMAKE_GENERATOR ${EP_CMAKE_GENERATOR}
   CMAKE_CACHE_ARGS
     -DCTK_SUPERBUILD:BOOL=OFF
     -DCTK_SUPERBUILD_BINARY_DIR:PATH=${CTK_BINARY_DIR}