Remove buffer release used as workaround

The extra buffer release was used as a workaround for a reactor core
issue and should have already been removed as part of
b3bc2d9.

Author: rstoyanchev

spring-core/src/main/java/org/springframework/core/io/buffer/DataBufferUtils.java

@@ -524,13 +524,9 @@ public void completed(Integer read, DataBuffer dataBuffer) {
 				long pos = this.position.addAndGet(read);
 				dataBuffer.writePosition(read);
 				this.sink.next(dataBuffer);
-				// It's possible for cancellation to happen right before the push into the sink
+				// onNext may have led to onCancel (e.g. downstream takeUntil)
 				if (this.disposed.get()) {
-					// TODO:
-					// This is not ideal since we already passed the buffer into the sink and
-					// releasing may cause something reading to fail. Maybe we won't have to
-					// do this after https://github.com/reactor/reactor-core/issues/1634
-					complete(dataBuffer);
+					complete();
 				}
 				else {
 					DataBuffer newDataBuffer = this.dataBufferFactory.allocateBuffer(this.bufferSize);
@@ -539,12 +535,12 @@ public void completed(Integer read, DataBuffer dataBuffer) {
 				}
 			}
 			else {
-				complete(dataBuffer);
+				release(dataBuffer);
+				complete();
 			}
 		}
 
-		private void complete(DataBuffer dataBuffer) {
-			release(dataBuffer);
+		private void complete() {
 			this.sink.complete();
 			closeChannel(this.channel);
 		}