Remove unused method to match up 4.1 branch

Motivation:

4 and 5 were diverged long time ago and we recently reverted some of the
early commits in master.  We must make sure 4.1 and master are not very
different now.

Modification:

Remove ChannelHandlerInvokerUtil.invokeWriteAndFlush(...) as it is not used anyway and not exists in 4.1

Result:

4.1 and master got closer.

transport/src/main/java/io/netty/channel/ChannelHandlerInvokerUtil.java

@@ -168,11 +168,6 @@ public final class ChannelHandlerInvokerUtil {
         }
     }
 
-    public static void invokeWriteAndFlushNow(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) {
-        invokeWriteNow(ctx, msg, promise);
-        invokeFlushNow(ctx);
-    }
-
     public static boolean validatePromise(
             ChannelHandlerContext ctx, ChannelPromise promise, boolean allowVoidPromise) {
         if (ctx == null) {