chore: remove mainnet and sepolia deploy configs

[jackchuma]

What changed? Why?

• Removed the checked-in deploy-config/mainnet.json and deploy-config/sepolia.json files so deployment-specific values are no longer maintained in the repository.
• Added an ignore rule for local deploy configs and updated the multiproof NoNitro guide to copy deploy-config/local.json to deploy-config/sepolia.local.json.
• Documented the required non-zero Base Sepolia l2BlockTime and l2GenesisTimestamp fields, and updated the guide commands to use the ignored local config.

Notes to reviewers

The local deployment config remains available as the starting schema; users must populate the current Sepolia values in their ignored copy before deploying.

How has it been tested?

• git diff --check
• git check-ignore -v deploy-config/sepolia.local.json
• Verified the documented Base Sepolia L2 timing values against the removed Sepolia config.

[cb-heimdall]

✅ Heimdall Review Status

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