refactor: remove Identifier listener in no-irregular-whitespace

[mdjermanovic]

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

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
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[x] Other, please explain:

In the no-irregular-whitespace rule, this removes an Identifier listener that is essentially a no-op since the removeInvalidNodeErrorsInIdentifierOrLiteral function has effects only when it's given a string literal node (the typeof node.value === "string" condition) or a regex literal node (the Boolean(node.regex) condition).

What changes did you make? (Give an overview)

Removed the listener and renamed function removeInvalidNodeErrorsInIdentifierOrLiteral to removeInvalidNodeErrorsInLiteral.

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

I checked the commits/PRs/issues history for this rule and have no idea why it was checking Identifier nodes. The listener is present since the original commit 77b0181. The only comment I could find about it (and generally about identifiers in regard to this rule) is https://github.com/eslint/eslint/pull/2381/files#r29104317.

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[nzakas]

If we can remove this without breaking any tests then chances are it's safe to remove.

The only thing I'm not sure of is if zero-width characters are valid identifier characters. It seems we don't have any tests for that, so feel free to merge.

[mdjermanovic]

if zero-width characters are valid identifier characters

I believe they're not valid identifier characters. I wrote this to check if any of the irregular characters this rule checks are allowed per prod-IdentifierName:

/[\p{ID_Start}\p{ID_Continue}$_\u200c\u200d]/u.test("\f\v\u0085\ufeff\u00a0\u1680\u180e\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200a\u200b\u202f\u205f\u3000\u2028\u2029"); // false

Per this test, neither of them is a valid identifier character. I'm not 100% sure that my test is correct, but either way even if the intent of the removed code was to allow irregular characters in identifiers, it wasn't working.