Remove default initval for Flat variables

[ricardoV94]

Description

This was brought up in #7348. I think this default was present before the support_point machinery was implemented and we used to take a random draw from the prior to decide on the initial point. That has long ceased to be the case.

Related Issue

• [ ] Closes #
• [x] Related to #7348

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [x] Included tests that prove the fix is effective or that the new feature works
• [x] Added necessary documentation (docstrings and/or example notebooks)
• [x] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [ ] New feature / enhancement
• [ ] Bug fix
• [ ] Documentation
• [x] Maintenance
• [ ] Other (please specify):

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📚 Documentation preview 📚: https://pymc--7379.org.readthedocs.build/en/7379/