doc/go1.18: remove residual TODOs

There doesn't seem anything that still needs to de done there.

Updates #47694.

Change-Id: I7909f566638332f3904d20a34f61d371af1d2da2
Reviewed-on: https://go-review.googlesource.com/c/go/+/371754


Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Trust: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>