Remove "arc feature", "arc branch", "arc bookmark", and significant chunks of obsolete marker code

Summary: Ref T13546. Moves away from the older workflows in favor of "arc branches", "arc bookmarks", and "arc work".

Test Plan: Grepped for affected symbols, didn't find any callers.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21337