Remove `.ExternalClass` rules in reset styles

In the HTML code generated by MJML, the first `<style>` tag contains the following rules :

```
.ExternalClass {
   width: 100%;
}

.ExternalClass * {
    line-height: 100%;
}
```

This was useful for Outlook.com up to circa 2016. The new versions of Outlook.com since 2016 no longer use the `.ExternalClass` prefix.

These styles should be safely removed from the generated HTML.
