modules: remove obsolete backwards-compat include dir.

No longer needed after all the "GL APIs in root namespace" compatibility
got removed.

modules/FindMagnum.cmake

@@ -344,12 +344,6 @@ if(NOT TARGET Magnum::Magnum)
     # Include directories
     set_property(TARGET Magnum::Magnum APPEND PROPERTY INTERFACE_INCLUDE_DIRECTORIES
         ${MAGNUM_INCLUDE_DIR})
-    # Some deprecated APIs use headers (but not externally defined symbols)
-    # from the GL library, link those includes as well
-    if(MAGNUM_BUILD_DEPRECATED AND MAGNUM_TARGET_GL)
-        set_property(TARGET Magnum::Magnum APPEND PROPERTY
-            INTERFACE_INCLUDE_DIRECTORIES ${MAGNUM_INCLUDE_DIR}/MagnumExternal/OpenGL)
-    endif()
 
     # Dependent libraries
     set_property(TARGET Magnum::Magnum APPEND PROPERTY INTERFACE_LINK_LIBRARIES