Fix image creation when HORIZON_IMAGES_ALLOW_UPLOAD == False

Replacing the source_type ChoiceField widget with HiddenInput causes the
form to fail (handle() is never called). Instead of this, just delete the
fields that are not applicable (the source selector and file upload fields).

Change-Id: I5016dcb7ee834120dc1f5b2b3ac66f00db3d390b