lib.modules: Define mergeOneOption in terms of mergeUniqueOption

lib/options.nix

@@ -128,11 +128,7 @@ rec {
     else if all isInt list && all (x: x == head list) list then head list
     else throw "Cannot merge definitions of `${showOption loc}'. Definition values:${showDefs defs}";
 
-  mergeOneOption = loc: defs:
-    if defs == [] then abort "This case should never happen."
-    else if length defs != 1 then
-      throw "The unique option `${showOption loc}' is defined multiple times. Definition values:${showDefs defs}"
-    else (head defs).value;
+  mergeOneOption = mergeUniqueOption { message = ""; };
 
   mergeUniqueOption = { message }: loc: defs:
     if length defs == 1