Remove /metadata endpoint

[lightwalker-eth]

Currently reviewing #919

Here it looks like we're removing any use of the /metadata endpoint. Is that right? If so let's fully remove all code related to that endpoint.

As a follow-up task, please also cleanup any related old code we no longer need. For example, can we remove?

• @/lib/ponder-metadata-provider
• the whole @ensnode/ponder-metadata package

[tk-o]

@lightwalker-eth we first need to refactor buildSubgraphGraphQLSchema function to requiring the metadataProvider: PonderMetadataProvider value on input.