doc: remove xreflabels from commits
75fcdd2ae2 and
85af628da5
xreflabels prevent references to the chapter numbers of sections id's.
It should only be used in specific cases.
Discussion: https://postgr.es/m/
8315c0ca-7758-8823-fcb6-
f37f9413e6b6@2ndquadrant.com
Backpatch-through: 9.5