Remove wrong import in FF AGIPD notebook

A fix for a reported bug at https://in.xfel.eu/redmine/issues/124675#change-463210

As the agipdlib.get_bias_voltage function was removed the code complains when trying to import it.

Description

How Has This Been Tested?

• being tested with the xfel-calibrate CL added by Marcin in the mentioned ticket

Relevant Documents (optional)

Types of changes

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

Checklist:

Reviewers

@schmidtp @roscar @danilevc