Remove unused ArcanistUncommittedChangesException class

Summary: Remove the ArcanistUncommittedChangesException class which is unused after D11843.

Test Plan: grep

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12515