[LPD] [Correct] Don't pass default snapshot_at=None to calibration_client

Description

The name of this parameter is changing to pdu_snapshot_at (ITDM/calibration_client!57), which will break the current code. Since we're passing None and that's the default anyway, the easy fix is not to pass it at all.

How Has This Been Tested?

It hasn't been tested (Maxwell is down), but it should be clear that it won't change any behaviour - snapshot_at is None either way.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.

Reviewers

@manettim @schmidtp @ahmedk