CommitMessage: do not use "Co-authored-by" and "Assisted-by" lines

Mention that we don't use "Co-authored-by` and "Assisted-by" lines,
similar to not using "Signed-off-by" lines.

Change-Id: I47f9c80e2dce532f04b8a265859bb415d589d0f0
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/840345
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: t hepudds <thepudds1460@gmail.com>
Commit-Queue: Cherry Mui <cherryyz@google.com>