Remove unused import

Author: ngocnhan-tran1996

oauth2/oauth2-resource-server/src/main/java/org/springframework/security/oauth2/server/resource/introspection/SpringOpaqueTokenIntrospector.java

@@ -20,7 +20,6 @@
 import java.net.URI;
 import java.net.URLEncoder;
 import java.nio.charset.Charset;
-import java.nio.charset.StandardCharsets;
 import java.time.Instant;
 import java.util.ArrayList;
 import java.util.Arrays;