compiler: remove doc comments from Zir

[mlugg]

This code was left over from the legacy Autodoc implementation. No component of the compiler pipeline actually requires doc comments, so it is a waste of time and space to store them in ZIR.

[andrewrk]

I considered this when looking into #131 but I concluded that doc comments would be desirable for #615.

You can make the call. It's possible for the compiler server we decide to get that information based on the AST instead. YAGNI principles would say go ahead and merge this.

[mlugg]

Yeah, I suspect #615 won't really benefit from this. I think I'll go ahead and merge it for now -- it'll be relatively easy to reintroduce if we ever do need the doc comments in ZIR.