fixed LessUtils::filterCommentless() did not respect less-comments

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |  #513

```less
// import (file.less)
```

Was not properly parsed, this fixes it.