haxe*: set licenses according to upstream opam file

Upstream specifies MIT and GPL2+ in its opam file, so we run with this.
There doesn't seem to have been any license change and I couldn't track
down the mentioned docs/license.txt.

pkgs/development/compilers/haxe/default.nix

@@ -104,7 +104,7 @@ let
       meta = with lib; {
         description = "Programming language targeting JavaScript, Flash, NekoVM, PHP, C++";
         homepage = "https://haxe.org";
-        license = with licenses; [ gpl2 bsd2 /*?*/ ];  # -> docs/license.txt
+        license = with licenses; [ gpl2Plus mit ]; # based on upstream opam file
         maintainers = [ maintainers.marcweber maintainers.locallycompact ];
         platforms = platforms.linux ++ platforms.darwin;
       };