(stable) Remove 'isPartial' parameter with no effect
Summary: Fixes T12536. Nothing reads this parameter; PhabricatorFile::newChunkedFile sets the isPartial flag automatically.
Test Plan: Grepped for isPartial.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T12536
Differential Revision: https://secure.phabricator.com/D17654