Remove support for Mistral in config-user-example.yml

[remi-kazeroni]

Description

This PR removes support for the offline machine DKRZ-Mistral as done in the config-user.yml file in ESMValGroup/ESMValCore#1620

• Related to New supercomputer at DKRZ: Levante #2631
• Link to documentation:

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[schlunma]

Thanks!!