Revert "add functionality for group_ids to User class"

This reverts commit 68789099

src/main/java/mops/gruppen2/domain/User.java

@@ -5,8 +5,6 @@ import lombok.Data;
 import lombok.EqualsAndHashCode;
 import lombok.Value;
 
-import java.util.List;
-
 @Value
 @AllArgsConstructor
 @EqualsAndHashCode(exclude = {"givenname", "familyname", "email"})
@@ -17,10 +15,4 @@ public class User {
     String givenname;
     String familyname;
     String email;
-
-    List<Long> group_ids;
-
-    public void addGroup(Long group_id){
-        group_ids.add(group_id);
-    }
 }