Clean up some code and comments in partbounds.c.
Do some minor cleanup for commit
c8434d64c: 1) remove a useless
assignment (in normal builds) and 2) improve comments a little.
Back-patch to v13 where the aforementioned commit went in.
Author: Etsuro Fujita
Reviewed-by: Alvaro Herrera
Discussion: https://postgr.es/m/CAPmGK16yCd2R4=bQ4g8N2dT9TtA5ZU+qNmJ3LPc_nypbNy4_2A@mail.gmail.com