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/applications/editors/gophernotes/default.nix

···
     homepage = "https://github.com/gopherdata/gophernotes";
     license = licenses.mit;
     maintainers = [ maintainers.costrouc ];
-    platforms = platforms.all;
   };
 }

pkgs/applications/networking/cluster/helm/plugins/helm-diff.nix

···
     inherit (src.meta) homepage;
     license = licenses.apsl20;
     maintainers = with maintainers; [ yurrriq ];
-    platforms = platforms.all;
   };
 }

pkgs/applications/networking/cluster/helm/plugins/helm-s3.nix

···
     inherit (src.meta) homepage;
     license = licenses.apsl20;
     maintainers = with maintainers; [ yurrriq ];
-    platforms = platforms.all;
   };
 }

pkgs/applications/version-management/git-and-tools/ghorg/default.nix

···
     homepage = "https://github.com/gabrie30/ghorg";
     license = licenses.asl20;
     maintainers = with maintainers; [ vidbina ];
-    platforms = platforms.all;
   };
 }

pkgs/development/libraries/boringssl/default.nix

···
   meta = with lib; {
     description = "Free TLS/SSL implementation";
     homepage    = "https://boringssl.googlesource.com";
-    platforms   = platforms.all;
     maintainers = [ maintainers.thoughtpolice ];
     license = with licenses; [ openssl isc mit bsd3 ];
   };

pkgs/development/tools/protoc-gen-go-grpc/default.nix

···
     description = "The Go language implementation of gRPC. HTTP/2 based RPC";
     license = licenses.asl20;
     maintainers = [ maintainers.raboof ];
-    platforms = platforms.all;
   };
 }

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

···
     homepage = "https://robustirc.net/";
     license = licenses.bsd3;
     maintainers = [ maintainers.hax404 ];
-    platforms = platforms.all;
   };
 }

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

···
     homepage = "https://github.com/mackerelio/mackerel-agent";
     license = licenses.asl20;
     maintainers = with maintainers; [ midchildan ];
-    platforms = platforms.all;
   };
 }