lib/types: Revert handling submodules for "either"

This reverts commit 0f0805b, because @nbp had concerns about whether
this would be a good idea and pointed out problems with this.

We currently do not have a case where "either" is used in conjunction
with submodules, but I'm reverting it anyway to prevent people from
adding options using that type in that way.

This is now being reviewed in #14053.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

lib/types.nix

@@ -246,25 +246,7 @@ rec {
     either = t1: t2: mkOptionType {
       name = "${t1.name} or ${t2.name}";
       check = x: t1.check x || t2.check x;
-      merge = loc: defs:
-             if all t1.check (getValues defs) then t1.merge loc defs
-        else if all t2.check (getValues defs) then t2.merge loc defs
-        else throw ( "The option `${showOption loc}' has conflicting"
-                   + " definitions for type either, in"
-                   + " ${showFiles (getFiles defs)}.");
-
-      getSubOptions = prefix: t1.getSubOptions prefix
-                           // t2.getSubOptions prefix;
-
-      getSubModules = concatLists (remove null [
-        t1.getSubModules
-        t2.getSubModules
-      ]);
-
-      substSubModules = m: let
-        maybeDef = def: r: if r == null then def else r;
-      in either (maybeDef t1 (t1.substSubModules m))
-                (maybeDef t2 (t2.substSubModules m));
+      merge = mergeOneOption;
     };
 
     # Obsolete alternative to configOf.  It takes its option