Remove file-by-file preference cookie

It's very likely that the fileViewMode cookie is redundant now that the setting is toggleable from the MR Changes tab.

The following discussion from !55931 (merged) should be addressed:

• @peterhegman continued a discussion: (+3 comments)

Ahh okay I think I see where the confusion was. The above steps work for me 👍. But I think the checkbox in the settings dropdown can still get out of sync with the profile settings because of the cookie. Take for example these steps:

1. Open my preferences, on 127.0.0.1:3000
2. Open an MR on 127.0.0.1:3000 in an incognito window
3. Check the Show one file at a time checkbox in the MR settings dropdown
4. Refresh my preferences page - the checkbox Show one file at a time on merge request's Changes tab should be checked
5. Uncheck the Show one file at a time on merge request's Changes tab on the preferences page and save
6. Refresh the MR page in the incognito window, the Show one file at a time checkbox is still checked when it should be unchecked

Maybe we should stop saving the cookie?

Proposal

We remove the cookie and all of the places that reference it.