Remove write pausing in Undertow response

Using the simple example shown in the ticket but switching from
Mono<String> to Flux<String> (and 5,000,000 onNext calls) shows that
constant pausing causes significant overhead and is not worth the
trouble vs ignoring the onWritePossible in REQUESTED state.

Issue: SPR-16702

spring-web/src/main/java/org/springframework/http/server/reactive/AbstractListenerWriteProcessor.java

@@ -162,7 +162,11 @@ public abstract class AbstractListenerWriteProcessor<T> implements Processor<T,
 	 * Invoked after the current data has been written and before requesting
 	 * the next item from the upstream, write Publisher.
 	 * <p>The default implementation is a no-op.
+	 * @deprecated originally introduced for Undertow to stop write notifications
+	 * when no data is available, but deprecated as of as of 5.0.6 since constant
+	 * switching on every requested item causes a significant slowdown.
 	 */
+	@Deprecated
 	protected void writingPaused() {
 	}
 
@@ -271,6 +275,7 @@ public abstract class AbstractListenerWriteProcessor<T> implements Processor<T,
 		},
 
 		RECEIVED {
+			@SuppressWarnings("deprecation")
 			@Override
 			public <T> void onWritePossible(AbstractListenerWriteProcessor<T> processor) {
 				if (processor.changeState(this, WRITING)) {

spring-web/src/main/java/org/springframework/http/server/reactive/UndertowServerHttpResponse.java

@@ -217,11 +217,6 @@ class UndertowServerHttpResponse extends AbstractListenerServerHttpResponse impl
 			return (dataBuffer.readableByteCount() == 0);
 		}
 
-		@Override
-		protected void writingPaused() {
-			this.channel.suspendWrites();
-		}
-
 		@Override
 		protected void writingComplete() {
 			this.channel.getWriteSetter().set(null);