Use pgstat_kind_infos to read fixed shared stats structs

Instead of needing to be explicit, we can just iterate the
pgstat_kind_infos array to find the memory locations to read into.

This was originally thought of by Andres in
5891c7a8ed8f2d3d577e7eea34dacff12d7b6bbd.

Not a fix, per se, but it does remove a comment. Perhaps the discussion
will just lead to someone deleting the comment, and keeping the code
as is. Either way, a win :).

--
Tristan Partin
Neon (https://neon.tech)