[caldav] Remove use of account_service_name account setting key

Instead of using this key to find the CalDAV service for an account,
look for a non-empty 'calendars' entry in the service settings. This
is a more reliable check and makes the account_service_name key
unnecessary.
Merge pull request #15 from nemomobile/change-account-service-check

[caldav] Remove use of account_service_name account setting key