Remove schoolbell

[Curtis Hovey]

Schoolbell was disabled in before launchpad 1.0 because the calendar was not a compelling feature. We maintain code for the obsolete feature years later. We can remove it.

Remove these filed and directories:
    ./lib/schoolbell
    ./lib/canonical/launchpad/components/cal.py
    ./doc/calendars.txt

Update these files.

    ./lib/lp/translations/browser/potemplate.py
        POTemplateFacets.calendar() method