Fix to removeLineClass to remove class from gutter markup

I found that gutter classes aren't being removed from the markup when `removeLineClass` is called. I've fixed the function and also amended the unit tests to support the fix:

I've updated the `addLineClass` unit test to ensure that gutter classes are removed from the markup and also tested that `line.handle.gutterClass` is cleared when `removeLineClass` is called for a gutter class.

I'm happy to amend the tests to make sure other line classes are also being removed from the markup.