nxproxy: drop the warning introduced by #55723

The tarball job fails when warnings are detected (and blocks channel).
And that's good, because `nix-env -qa` also gets these warnings.

I'm afraid we still don't have a good way to deprecate attributes,
exactly because the inability to distinguish these "listing actions"
from explicit usage (direct or transitive).

pkgs/top-level/aliases.nix

@@ -209,7 +209,7 @@ mapAliases ({
   nilfs_utils = nilfs-utils; # added 2018-04-25
   nmap_graphical = nmap-graphical;  # added 2017-01-19
   nologin = shadow; # added 2018-04-25
-  nxproxy = lib.warn "nxproxy will be removed soon, use `nx-libs` instead" nx-libs; # added 2019-02-15
+  nxproxy = nx-libs; # added 2019-02-15
   opencascade_oce = opencascade; # added 2018-04-25
   opencl-icd = ocl-icd; # added 2017-01-20
   openexr_ctl = ctl; # added 2018-04-25