Removed willProcessRequest from DifferentialRevisionLandController

Summary: Ref T8628.

Test Plan: Landed a revision through the web UI

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley, yelirekim

Maniphest Tasks: T8628

Differential Revision: https://secure.phabricator.com/D16572