chore: delete mumbai network

[antazoey]

What I did

Mumbai has been gone since April 8th so it is time to delete this network. Also, some other plugins already have..

How I did it

How to verify it

Checklist

• [ ] Passes all linting checks (pre-commit and CI jobs)
• [ ] New test cases have been added and are passing
• [ ] Documentation has been updated
• [ ] PR title follows Conventional Commit standard (will be automatically included in the changelog)