Remove Serializable from OAuth2User

Fixes gh-4514

oauth2/oauth2-core/src/main/java/org/springframework/security/oauth2/core/user/OAuth2User.java

@@ -19,7 +19,6 @@ import org.springframework.security.core.AuthenticatedPrincipal;
 import org.springframework.security.core.Authentication;
 import org.springframework.security.core.GrantedAuthority;
 
-import java.io.Serializable;
 import java.util.Collection;
 import java.util.Map;
 
@@ -47,7 +46,7 @@ import java.util.Map;
  * @see DefaultOAuth2User
  * @see AuthenticatedPrincipal
  */
-public interface OAuth2User extends AuthenticatedPrincipal, Serializable {
+public interface OAuth2User extends AuthenticatedPrincipal {
 
 	Collection<? extends GrantedAuthority> getAuthorities();