Remove JDK 9 workarounds etc

See gh-17778

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompDecoder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,7 +17,6 @@
 package org.springframework.messaging.simp.stomp;
 
 import java.io.ByteArrayOutputStream;
-import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
@@ -134,11 +133,7 @@ public class StompDecoder {
 	private Message<byte[]> decodeMessage(ByteBuffer byteBuffer, @Nullable MultiValueMap<String, String> headers) {
 		Message<byte[]> decodedMessage = null;
 		skipEol(byteBuffer);
-
-		// Explicit mark/reset access via Buffer base type for compatibility
-		// with covariant return type on JDK 9's ByteBuffer...
-		Buffer buffer = byteBuffer;
-		buffer.mark();
+		byteBuffer.mark();
 
 		String command = readCommand(byteBuffer);
 		if (command.length() > 0) {
@@ -176,7 +171,7 @@ public class StompDecoder {
 						headers.putAll(map);
 					}
 				}
-				buffer.reset();
+				byteBuffer.reset();
 			}
 		}
 		else {
@@ -357,8 +352,7 @@ public class StompDecoder {
 					throw new StompConversionException("'\\r' must be followed by '\\n'");
 				}
 			}
-			// Explicit cast for compatibility with covariant return type on JDK 9's ByteBuffer
-			((Buffer) byteBuffer).position(byteBuffer.position() - 1);
+			byteBuffer.position(byteBuffer.position() - 1);
 		}
 		return false;
 	}