Remove NEXT_PUBLIC_MACI_START_BLOCK

[kittybest]

Replace this argument with start blocks of deployed MACI contract, stored in the args in the localStorage.

• remove this arg in .env.example, env.js, and config.ts
• return this with maciAddress in the MaciService.getMaciAddress
• update getMaciAddress test
• update related documents (MaciContext)

[kittybest]

close by PR #121