Removed the use of uForum:ResolveLinks and uForum:CleanBBCode in the xslt rendering topic and comments

Without fully understanding what those methods do, I believe they are
somewhat legacy and irrelevant for the new markdown editor. And since
they were messing with the raw markdown text I simply removed the use of
them :)

Oh, and I also removed the reference to MarkdownDeep, as it's unused on
the OurUmbraco.Site project.