Remove redundant check in `add_config_mismatch_label` function

There is no need to check if the label is present to remove it. It has already been done by the function remove_labels_from_merge_request internally. Another check outside the function is redundant.

This MR also updates the test because now it requires mocking other attributes of the project and the merge request.

Signed-off-by: Julio Faracco jfaracco@redhat.com