Describe the bug
Update does not remove introductory image.

In Consultation's edit form, checking "remove introductory image" does not remove it. It should do it!

A quick look into UpdateConsultation form, remove_introductory_image attribute is missing.

To Reproduce
Steps to reproduce the behavior:

1. Go to Consultations edit form
2. Check "remove introductory image"
3. Submit form
4. Image has not been removed

Expected behavior
Image should be removed

Extra data (please complete the following information):

• Decidim Version: [e.g. 0.11.1]