Fix Imports of OAuth2AccessTokenResponse

Issue: gh-5474

oauth2/oauth2-core/src/main/java/org/springframework/security/oauth2/core/endpoint/OAuth2AccessTokenResponse.java

@@ -21,7 +21,6 @@ import org.springframework.security.oauth2.core.OAuth2RefreshToken;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.StringUtils;
 
-import java.time.Duration;
 import java.time.Instant;
 import java.util.Collections;
 import java.util.Map;