nix-support/build/outputs: don't expose rl-next.user

I'm not sure why this wasn't an issue before, but the generation of
jobset constituents in Hydra trips over this. The Hydra-convention is to
have the `system`-attribute last anyways, not in-between.

Since this is a single attribute, get rid of it for the sake of
simplicity.

Change-Id: I80ef1b92ed158c8712ee899534ce82130614f338

nix-support/build/outputs.nix

@@ -153,9 +153,7 @@ lib.fix (self: {
             LANG=C.UTF-8 ${lib.getExe pkgs.build-release-notes} --change-authors ${../../doc/manual/change-authors.yml} ${dir} >$out
           '';
       in
-      {
-        user = rl-next-check "rl-next" ../../doc/manual/rl-next;
-      }
+      rl-next-check "rl-next" ../../doc/manual/rl-next
     );
 
     # Completion tests for the Nix REPL.