[Misc] Cleanup unused cmake util. (#6084)

Co-authored-by: Ubuntu <ubuntu@ip-172-31-28-63.ap-northeast-1.compute.internal>

cmake/util/Util.cmake

@@ -44,16 +44,3 @@ macro(dgl_option variable description value)
     unset(${variable} CACHE)
   endif()
 endmacro()
-
-function(assign_source_group group)
-    foreach(_source IN ITEMS ${ARGN})
-        if (IS_ABSOLUTE "${_source}")
-            file(RELATIVE_PATH _source_rel "${CMAKE_CURRENT_SOURCE_DIR}" "${_source}")
-        else()
-            set(_source_rel "${_source}")
-        endif()
-        get_filename_component(_source_path "${_source_rel}" PATH)
-        string(REPLACE "/" "\\" _source_path_msvc "${_source_path}")
-        source_group("${group}\\${_source_path_msvc}" FILES "${_source}")
-    endforeach()
-endfunction(assign_source_group)