Remove file-by-file preference cookie

What does this MR do?

For #327162 (closed)

This MR removes the usage of the File-By-File (e.g. "One file at a time") cookie since the initial setting already comes from the user's preferences, and toggling the checkbox in the Diffs UI also toggles the user's preference.

Because the utils/preferences.js#fileByFile helper method is essentially just a wrapper that merges the "default" preference with the cookie, that whole file (the method is the only content) and its test suite are also deleted.

preferences.js

I think there's a lot of value at GitLab in having defined modules like diffs/utils/preferences.js as "the place" to put preferences helpers. Momentum carries a very significant weight in review, and being able to say "Why don't we put all our preferences getters over in preferences.js?" is really helpful. It pains me a bit to remove the whole module so it's not even there to reference.

Screenshots or Screencasts (strongly suggested)

N/A. There's no UI change.

How to setup and validate locally (strongly suggested)

The only way to test this locally is to have a bit of an odd setup that you can't really get in the UI any more.

Basically:

1. Have your user preference ("Show one file at a time on merge request's Changes tab") set to something (it doesn't matter, checked or unchecked).
2. Set the cookie (fileViewMode) to the opposite value (you can see the two options here in the removed constants: single or all)

On the current master branch, your cookie setting will override the preference setting.

Note that if you toggle the setting using the Cog dropdown > "Show one file at a time" checkbox in the MR Changes tab, the cookie and the user preference will re-sync.

On this MR's branch, the cookie is ignored.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.