[GH2][Correct] Move false warning and disable gain correction as printed.

Description

This is related to https://git.xfel.eu/calibration/pycalibration/-/issues/74

Gain correction should be disabled only when RelativeGainGotthard2 is not retrieved. This is what already happens, but the warning would say otherwise if BadPixelsFFGotthard2 was the only non retrieved constant.

This fixes the warning message and disables the gain correction as announced.

How Has This Been Tested?

• Against reference runs.

Relevant Documents (optional)

Types of changes

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

Checklist:

Reviewers

@kluyvert @schmidtp