wasm: remove alias to unbreak the channel

Nixpkgs' channel currently can't move forward so long as there is a
trace in evaluating the top-level arguments. Which means that it isn't
possible to add a warning message to warn users of future package
removal.

So the only way forward appears to be just removing the alias
altogether.

(cherry picked from commit b4133eb)

nixos/doc/manual/release-notes/rl-1903.xml

@@ -130,6 +130,13 @@
       were removed. They were never used for anything and can therefore safely be removed.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     Package <literal>wasm</literal> has been renamed <literal>proglodyte-wasm</literal>. The package
+     <literal>wasm</literal> will be pointed to <literal>ocamlPackages.wasm</literal> in 19.09, so
+     make sure to update your configuration if you want to keep <literal>proglodyte-wasm</literal>
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 

pkgs/top-level/aliases.nix

@@ -322,13 +322,8 @@ mapAliases ({
   xpraGtk3 = xpra; # added 2018-09-13
   youtubeDL = youtube-dl;  # added 2014-10-26
 
-  # added 2018-10-16
-  # TODO(ekleog): remove after 19.03 branch-off
-  # TODO(ekleog): add ‘wasm’ alias to ‘ocamlPackages.wasm’ after 19.09
+  # TODO(ekleog): add ‘wasm’ alias to ‘ocamlPackages.wasm’ after 19.03
   # branch-off
-  wasm = lib.warn
-    "‘wasm’ package has been renamed ‘proglodyte-wasm’, and will be dropped in the next release"
-    proglodyte-wasm;
 
   # added 2017-05-27
   wineMinimal = winePackages.minimal;