crash_sender: remove obsolete is_on_3g(), improve comment

BUG=None
TEST=`cbuildbot x86-generic-full` passed

Change-Id: I49e259d42b1edaf0fb43ee912fa99799ca87f08c
Reviewed-on: https://chromium-review.googlesource.com/200440
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed