remove duplicated check for allowed reminders and availability

these checks seem to be unnecessary as `prepareReminders()` and `prepareAvailability()`
already remove unavailable reminder and availabilities, and while doing so checked for
bad strings delivered by the calendar provider.

by relying only on these functions, we also fix #1292, which occurred because EteSyncs
offline calendar provided said bad string for availability.

Change-Id: Ic7af69fc29f9c62ea6f3da85069db19b67134380