Revert "perlPackages: add default meta.mainProgram (#176398)"

This reverts commit ff7b216dcf.

pkgs/development/perl-modules/generic/default.nix

@@ -35,7 +35,6 @@ lib.warnIf (attrs ? name) "builtPerlPackage: `name' (\"${attrs.name}\") is depre
 (let
   defaultMeta = {
     homepage = "https://metacpan.org/release/${lib.getName attrs}"; # TODO: phase-out `attrs.name`
-    mainProgram = attrs.pname or (builtins.parseDrvName attrs.name).name;
     platforms = perl.meta.platforms;
   };