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