Remove modmailpro module

[eritbh]

It's no longer possible to access the old modmail interface at /message/moderator or /r/sub/message/inbox - both redirect to new modmail. Since there's no way to access the old modmail interface anymore, I'm pretty sure the (non-new) ModMailPro module can no longer be accessed and is safe to remove?

I was working on another issue (#220, #648) and realized that I probably don't need to be updating the code MMP uses for color-coding subs since it will never actually happen anymore.