treewide: go: inherit platforms instead of using platforms.all

`buildGoModule` and `buildGoPackage` by default inherit the `platforms`
from go. That seems better than explicitly configuring `platforms.all`.

There are also many packages that specify 'linux + darwin' - this is
even suggested in the documentation. We might also want to update those,
but let's do the noncontroversial change first.

pkgs/servers/irc/robustirc-bridge/default.nix

@@ -24,6 +24,5 @@ buildGoModule rec {
     homepage = "https://robustirc.net/";
     license = licenses.bsd3;
     maintainers = [ maintainers.hax404 ];
-    platforms = platforms.all;
   };
 }

pkgs/servers/monitoring/mackerel-agent/default.nix

@@ -37,6 +37,5 @@ buildGoModule rec {
     homepage = "https://github.com/mackerelio/mackerel-agent";
     license = licenses.asl20;
     maintainers = with maintainers; [ midchildan ];
-    platforms = platforms.all;
   };
 }