Chore: remove currentFilename prop from Linter instances (refs #9161)

[not-an-aardvark]

What is the purpose of this pull request? (put an "X" next to item)

[x] Other, please explain:

What changes did you make? (Give an overview)

This removes the undocumented currentFilename property from Linter instances.

This relies on #9218. The tests will pass after #9218 is merged.

Is there anything you'd like reviewers to focus on?

Nothing in particular

[eslintbot]

LGTM

[mention-bot]

@not-an-aardvark, thanks for your PR! By analyzing the history of the files in this pull request, we identified @gyandeeps, @nzakas and @mysticatea to be potential reviewers.

[ilyavolodin]

I'm still a bit weary of those kind of changes. Even those those methods are not documented, their removal might be considered a breaking change, since it might break some existing tools.

[not-an-aardvark]

¯\_(ツ)_/¯ I'm fine with it because we have the "use at your own risk" warning for undocumented APIs.

[platinumazure]

Possible compromise: Could we consider this an Update rather than a Chore and make it semver-minor?

[eslintbot]

LGTM

[not-an-aardvark]

I'm fine with including this in a semver-minor release as a compromise, although I don't see how it would end up there based on our semver policy. To me, it seems like one of the following is true:

1. There are integration developers ignoring the "use at your own risk" warning, and we're willing to accomodate them to avoid breaking builds, so we consider it a breaking change, or:
2. (1) isn't true, so this is just an internal refactor, which is semver-patch.

Basically, I don't see how this fits under Update: in either case.