Fix an annoying warning from GTK on Ubuntu 14.04.

Timer objects evaporate when our timer_trigger callback is called, and
therefore we should not remember their ids beyond that time and
attempt to cancel them later. Previous versions of GTK silently
ignored us doing that, but upgrading to Ubuntu Trusty has given me a
version of GTK that complains about it, so let's stop doing it.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10181 cda61777-01e9-0310-a592-d414129be87e