Fix _n_noop() and _nx_noop()

[westonruter]

These don't take a $number arg.

See #50 and #136.

[westonruter]

@GaryJones any concerns with this?

[GaryJones]

@westonruter Assuming the tests pass, then it would be fine, but I do have two suggestions.

Take a test string like:

_n_noop( 'I have a cat.', 'I have cats.', "illegal $string" );

I'd like to see test string variants for each relevant i18n function that include:

• a %s placeholder in both the single and plural strings - it shouldn't make any difference to the logic, but strings with placeholders for the number (i.e. 'I have %s cat.', 'I have %s cats.') is more like real-world usage. %d may be a better choice, but I've got a feeling I read something about there being a drawback to trying to force an int compared to allowing any string.

• a variable (or at least a dollar sign) in a single string text domain:

  _n_noop( 'I have a cat.', 'I have cats.', 'illegal-$string' ); // OK
  

  I don't know what the rules are regarding dollar signs in text domains, and that's probably a set of separate checks, but if so, it shouldn't trigger the same error as a straight variable or a variable inside double-quotes.