Fix new Sonar smells

**Cherry-pick to `6.0.x`**

Author: artembilan

spring-integration-mqtt/src/main/java/org/springframework/integration/mqtt/core/AbstractMqttClientManager.java

@@ -120,7 +120,7 @@ protected long getCompletionTimeout() {
 	 * @param completionTimeout The timeout.
 	 * @since 6.0.3
 	 */
-	public synchronized void setDisconnectCompletionTimeout(long completionTimeout) {
+	public void setDisconnectCompletionTimeout(long completionTimeout) {
 		this.disconnectCompletionTimeout = completionTimeout;
 	}
 

spring-integration-mqtt/src/main/java/org/springframework/integration/mqtt/inbound/AbstractMqttMessageDrivenChannelAdapter.java

@@ -194,7 +194,7 @@ public String[] getTopic() {
 	 * @param completionTimeout The timeout.
 	 * @since 5.1.10
 	 */
-	public synchronized void setDisconnectCompletionTimeout(long completionTimeout) {
+	public void setDisconnectCompletionTimeout(long completionTimeout) {
 		this.disconnectCompletionTimeout = completionTimeout;
 	}