Remove obsolete Calendar event date storage fields

Summary: Ref T11809. These have been replaced with more flexible storage that accommodates a wider range of behaviors, including those in the ICS format and RRULEs.

Test Plan:

• Ran migration.
• Viewed, created, edited events.
• Grepped for all removed names/symbols.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11809

Differential Revision: https://secure.phabricator.com/D16789