caprine: remove redundant mainProgram specification

Remove per-system meta.mainProgram specification.

So far, both Linux and Darwin version of Caprine names its executable
`caprine`.

Use `caprine` as the default mainProgam. Systems added in the future
could override this with custom meta attributes.

pkgs/applications/networking/instant-messengers/caprine-bin/build-from-appimage.nix

@@ -43,6 +43,5 @@ in
 
   meta = metaCommon // {
     platforms = [ "x86_64-linux" ];
-    mainProgram = "caprine";
   };
 })

pkgs/applications/networking/instant-messengers/caprine-bin/build-from-dmg.nix

@@ -30,6 +30,5 @@ stdenvNoCC.mkDerivation {
 
   meta = metaCommon // {
     platforms = with lib.platforms; darwin;
-    mainProgram = "caprine";
   };
 }