Trim transaction overload when converting events from all-day and back

Summary: Closes T8136, Trim transaction overload when converting events from all-day and back

Test Plan: Create new event, save, edit, change to all-day, save, remove all-day flag, save. Feed should not show "end date changed" transaction

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8136

Differential Revision: https://secure.phabricator.com/D12781