jslint: remove whitespaces using simple regex

Small speed improvement to the default lintjs extension. Replacing obsolete for loop with a simple regex to remove whitespaces, if a line contains only whitespace.
`\f` stands for form feed. I don't know if it is necessary.

> Only the characters "space" (U+0020), "tab" (U+0009), "line feed" (U+000A), "carriage return" (U+000D), and "form feed" (U+000C) can occur in whitespace.

As per [http://www.w3.org/TR/css3-selectors/#whitespace](http://www.w3.org/TR/css3-selectors/#whitespace)

Maybe `\f` is catched by `$`. I'm checking it later ...