Fix checkstyle violation

Author: snicoll

spring-websocket/src/test/java/org/springframework/web/socket/server/support/WebSocketHttpRequestHandlerTests.java

@@ -40,7 +40,6 @@
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyNoMoreInteractions;
-import static org.mockito.Mockito.when;
 
 /**
  * Unit tests for {@link WebSocketHttpRequestHandler}.