Remove unused 'mirrored' icons

[mikewheaton]

Once this PR merges, we'll be using CSS to mirror all of the directional icons (e.g. arrows) for right-to-left users.

With that in place, I don't see a need for icons like HelpMirrored and MailReplyMirrored, as these would only be shown in RTL scenarios and add confusion and file size to the icon font.

We can remove those icons from the font on the CDN and from Fabric's classes.