Code Cleanup: GcCalendarModel

.. wow, nasty memory leak fixed. Leaked memory every
time a ride was selected or calendar was refreshed!

.. tbf it was marked with a 'XXX' memory leak comment
but really lazy to leave it there.

.. was also highlighted by valgrind, which is promising,
since I've been using it to hunt down any memory managment
issues -- most of them are within 3rd party libraries tho.
Merge branch 'master' of github.com:/GoldenCheetah/GoldenCheetah