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.
@redmunds I think the issue there is that `\s` matches \n chars too, so we lose the ability to scope the regexp to just one line.  E.g. `"foo\nbar\nbaz".match(/^bar\sbaz$/gm)` --> `"bar\nbaz"`.  So what we really want is `\s` _except_ for \n, but I don't think there's a syntax in JS regexps to express that.

Since this is really just intended to screen out the blank-line indent that Brackets automatically generates, it seems safe enough to only look at spaces & tabs though, since we never insert anything else.