Removed the xpm cycle time digits in gcylc dot view.

This is to support #119 as the gcylc dot view can't currently display ISO8601 cycle times.

I don't think making the graphical digits compatible with ISO time formats is worth the effort - this is really another quirky hold-over from my early GUI that should be removed anyway.

This change is purely internal to the dot view code, there should not be any problem of compatibility with existing suites or daemons.

@benfitzpatrick - please review.