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