Adjust comment atop ExecShutdownNode.
After commits
a315b967cc and
b805b63ac2, part of the comment atop
ExecShutdownNode is redundant. Adjust it.
Author: Amit Kapila
Backpatch-through: 10 where both the mentioned commits are present.
Discussion: https://postgr.es/m/
86137f17-1dfb-42f9-7421-
82fd786b04a1@anayrat.info