Restore fix undid by https://github.com/grpc/grpc/pull/5893

That PR wasn’t tested nor reviewed adequately.

src/objective-c/GRPCClient/GRPCCall.m

@@ -378,7 +378,7 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
       [strongSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
                                                       code:GRPCErrorCodeUnavailable
                                                   userInfo:@{NSLocalizedDescriptionKey: @"Connectivity lost."}]];
-      [[GRPCHost hostWithAddress:strongSelf->_host] discardChannel:channel];
+      [[GRPCHost hostWithAddress:strongSelf->_host] disconnect];
     }
   }];
 }