groupApi removeMember remove subgroup and projects member

[CHYhave]

1. add subgroup member
2. add group member
3. remove group member
   expect
   group member has been removed and keep subgroup member
   actual
   subgroup also has been removed

[CHYhave]

test case:
`
@test
public void removeMember() throws GitLabApiException, MalformedURLException {
Integer groupId = 3, userId = 2;
Integer subGroupId = 5;
GitLabApi gitLabApi = new GitLabApi("http://10.10.102.202:33200/", "glpat-mW-1QqaqTquBGdGt8UY6");
GroupApi groupApi = gitLabApi.getGroupApi();

    Member groupMember = groupApi.getOptionalMember(groupId, userId, true).orElse(null);
    if (groupMember == null) {
        groupApi.addMember(groupId, userId, AccessLevel.MAINTAINER);
        groupMember = groupApi.getOptionalMember(groupId, userId).orElse(null);
    }
    Assert.assertNotNull(groupMember);

    Member subGroupMember = groupApi.getOptionalMember(subGroupId, userId).orElse(null);
    if (subGroupMember == null) {
        groupApi.addMember(subGroupId, userId, AccessLevel.MAINTAINER);
        subGroupMember = groupApi.getOptionalMember(subGroupId, userId).orElse(null);
    }
    Assert.assertNotNull(subGroupMember);

    Group group = groupApi.getGroup(groupId);
    groupApi.removeMember(group.getId(), userId);
    groupMember = groupApi.getOptionalMember(groupId, userId).orElse(null);
    Assert.assertNull(groupMember);
    subGroupMember = groupApi.getOptionalMember(subGroupId, userId).orElse(null);
    Assert.assertNotNull("sub group member has been removed", subGroupMember);
}

`