Remove dedicated "Jump Nav" navigation element

Summary: Fixes T5175. Not sure if I cleaned out everything, but this seemed like a reasonable first pass. Attempted to delete all code that belonged to Jump Nav feature only.

Test Plan: Open phabricator homepage, verify Jump Nav element is gone, verify the Search bar still autocompletes and jumps to shortcuts.

Reviewers: chad, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5175

Differential Revision: https://secure.phabricator.com/D9301