R1:7cd4e70ef2aa

Remove DifferentialFieldSelector

Summary: Ref T2222. Gets rid of DifferentialFieldSelector, favoring `differential.fields`.

Test Plan: `grep`

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8472