Remove uses of the "merge" option attribute

It's redundant because you can (and should) specify an option type, or
an apply function.

nixos/modules/system/activation/top-level.nix

@@ -172,9 +172,9 @@ in
     };
 
     system.extraSystemBuilderCmds = mkOption {
+      type = types.lines;
       internal = true;
       default = "";
-      merge = concatStringsSep "\n";
       description = ''
         This code will be added to the builder creating the system store path.
       '';

nixos/modules/system/boot/kernel.nix

@@ -117,6 +117,7 @@ in
     };
 
     system.modulesTree = mkOption {
+      type = types.listOf types.path;
       internal = true;
       default = [];
       description = ''
@@ -124,7 +125,6 @@ in
         built outside of the kernel.  Combine these into a single tree of
         symlinks because modprobe only supports one directory.
       '';
-      merge = mergeListOption;
       # Convert the list of path to only one path.
       apply = pkgs.aggregateModules;
     };

nixos/modules/system/boot/systemd-unit-options.nix

@@ -319,8 +319,7 @@ rec {
     options = mkOption {
       default = "";
       example = "noatime";
-      type = types.string;
-      merge = concatStringsSep ",";
+      type = types.commas;
       description = "Options used to mount the file system.";
     };