ical feed link can't be disabled

Problem/Motivation

On disabling a ical feed view display we should expect that the ical feed link disapear but it doesn't.
The only way to make it disapear is to delete the view display. To bad if you expect to enable it later.

Steps to reproduce

1. Create a new view with a page providing a path, and an ical feed. Select ical field.
2. On displaying the view page (tiping the path on your browser address bar, Not on the live preview !) The ical icon should appear at the very bottom and left region on the page.
3. Now disable the this feed display :
4. the ical feed icon is still visible but it should not.
5. Now delete this feed display :
6. the ical feed icon is is gone. You did it but now your view display is lost.

Proposed resolution

the patch provided below fixes this issue, at least for me.

Remaining tasks

review

User interface changes

none

API changes

none

Data model changes

none