Multires: Make topology cache to work with copy-on-write

Allows to preserve topology cache even when copy-on-write is
fully re-copying the object. For example, toggling edit mode
in and out does no longer causes full topology to be re-created.

source/blender/blenloader/intern/readfile.c

@@ -5786,10 +5786,6 @@ static void direct_link_modifiers(FileData *fd, ListBase *lb)
 				}
 			}
 		}
-		else if (md->type == eModifierType_Multires) {
-			MultiresModifierData *mmd = (MultiresModifierData *)md;
-			mmd->subdiv = NULL;
-		}
 	}
 }