[Weather Calendar] Weather info source never freed

The priv->source was allocated, but never freed, especially after
the weather backend was freed, which might cause use-after-free
in the finished_retrieval_cb() callback, when the weather backend
was freed during the data update.

This might be the cause of a crash reported at:
https://bugzilla.redhat.com/show_bug.cgi?id=1065796