Remove "cut point commit identifier" from Releeph
Summary: Ref T3656. Releeph denormalizes branch cut point identifiers into Branch objects, but this information isn't useful or used for sorting, filtering, or enforcing unique constraints. Instead, derive it via noramlized pathways from the cutPointCommitPHID.
Test Plan: Ran storage upgrade. Ran releephwork.getbranch and releeph.getbranches. Grepped for cutPointCommitIdentifier.
Reviewers: btrahan
Reviewed By: btrahan
CC: LegNeato, aran
Maniphest Tasks: T3656
Differential Revision: https://secure.phabricator.com/D6636