R1:21709a2bbc1a

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