[AGIPD][DARK] fix gain mode validation for FIXED_GAIN operation mode

A fix for selecting Fixed_Gain operation mode for fixed gain dark runs. Related to this ticket https://in.xfel.eu/redmine/issues/125424

Description

The Dark notebook didn't depend much on selecting the correct operation mode, to avoid operator mistakes by requesting darks for fixed gain runs with ADAPTIVE GAIN operation mode.

Ironically when the operator selected the correct configuration for the runs. The notebook failed with the wrong conditions.

This MR fixes this mess.

• Removing this undefined runs parameter.
• Fix the conditions in case FIXED_GAIN is selected.
• Move all the validation for gain_mode parameter in validate_gain_modes

How Has This Been Tested?

Running the notebook interactively for the runs mentioned in the problem ticket

Relevant Documents (optional)

Types of changes

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

Checklist:

Reviewers

@schmidtp @tmichela @hammerd