mig-generated server dispatch routines used to not clear this field in
reply messages prepared from request messages. This oversight was
corrected in the migcom in bootstrap_cmds-96 (macOS 10.12 and Xcode
8.0). Maybe someone at Apple saw the admonishing comment that we had
left here. This comment can now be removed.
Change-Id: I73d965705a2ff5788afb59dd8ecdf4afe58ee47e
Reviewed-on: https://chromium-review.googlesource.com/465687
Reviewed-by: Robert Sesek <rsesek@chromium.org>