Remove ENSRainbow.io default API url in ENSIndexer

[shrugs]

the .env.local.example should not provide a default ENSRAINBOW_API value and should encourage the user to run ENSRainbow locally or on the same private network for performance reasons.

• update ensindexer/.env.local.example
  • can include in the comments a note like "while not recommended, you can temporarily configure ENSIndexer to use the public ENSRainbow API here:" or whatever
• update any documentation references
• ENSIndexer should panic if no ENSRAINBOW_API endpoint configured (or is not valid url)