[PATCH] Minor cleanups of BRIN code

Hi list,

I was messing around a bit with BRIN code and found some cleanups to be made:
1. Remove dead code in brin_form_tuple
2. Add missing Datum conversion macros
3. Use AttrNumber type in places using 1-based attribute numbers

Though it's not hard to find cases all over source code similar to (2)
and (3), are these kinds of cleanups considered useful?

Regards,
Marti Raudsepp