Remove the hard-coded list of chains from the indexer

[matthewdarwin]

Ref: https://github.com/graphprotocol/indexer/issues/915

A lot more new chains are coming to the network. We cannot maintain a list of supported chains in multiple places. List of supported chains should be derived from the network subgraph or other canonical location.

[matthewdarwin]

See also https://github.com/graphprotocol/indexer/issues/905

[dwerner]

Speaking with @alex-pakalniskis - we should give more thought to where the source of truth lives in the network. Setting as blocked.

[matthewdarwin]

@dwerner on chain, right? (cc @alex-pakalniskis )