Chore: Don't require an issue reference in check-commit npm script

[not-an-aardvark]

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

[ ] Documentation update
[ ] Bug fix ([template](https://github.com/eslint/eslint/blob/master/templates/bug-report.md))
[ ] New rule ([template](https://github.com/eslint/eslint/blob/master/templates/rule-proposal.md))
[ ] Changes an existing rule ([template](https://github.com/eslint/eslint/blob/master/templates/rule-change-proposal.md))
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[x] Other, please explain:

If the item you've check above has a template, please paste the template questions here and answer them.

Please check each item to ensure your pull request is ready:

• I've read the pull request guide
• I've included tests for my change
• I've updated documentation for my change (if appropriate)

What changes did you make? (Give an overview)

This removes the check for an issue reference in the npm run check-commit script, since that is no longer required by eslintbot or the docs. (discussed in Gitter)

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

Nothing in particular.

[eslintbot]

LGTM

[platinumazure]

For any reviewers: I had recommended submitting the PR with no tests because there don't seem to be existing tests. If we want tests around Makefile.js functionality, my preference would be that we just include checkGitCommit tests in this PR and create a separate issue/PR for adding test coverage elsewhere in that file.

[platinumazure]

@not-an-aardvark Thanks for submitting this! Unfortunately you need to remove the ISSUE_REGEX declaration as well since the variable is unused. I'll take another look when Travis and AppVeyor are passing 😄

[eslintbot]

LGTM

[not-an-aardvark]

Whoops, that's what I get for not running the tests locally. Fixed.

[platinumazure]

LGTM, waiting for others to review.

[nzakas]

LGTM. Thanks!