Do not delay marking messages as read when user doesn't want a delay

Thanks to Ed Tomlinson <edtoml@gmail.com> for reporting this on IRC. The
delay is actually already implemented within the MsgListView, so
delaying stuff even further here makes little sense now.

The 200ms was some arbitrary change made yars ago:

commit 6892cd36
Author: Jan Kundrát <jkt@flaska.net>
Date:   Wed May 30 21:51:01 2012 +0200

    GUI: mark visited messages as read almost instantly

Change-Id: Idbe3a15d18ac1f0c881d0dc789bff2c91f6ae13b