ir-lv2: remove mainProgram

The mainProgram was set in a treewide effort but does not actually
reflect the main program.

From
https://github.com/NixOS/nixpkgs/blob/master/pkgs/README.md#meta-attributes:

meta.mainProgram must be set to the name of the executable which
facilitates the primary function or purpose of the package.

Packages which are not primarily used for a single executable do not need to set meta.mainProgram.

pkgs/by-name/ir/ir-lv2/package.nix

@@ -49,6 +49,5 @@ stdenv.mkDerivation (finalAttrs: {
     license = lib.licenses.gpl2;
     maintainers = [ lib.maintainers.magnetophon ];
     platforms = lib.platforms.linux;
-    mainProgram = "convert4chan";
   };
 })