Remove fallback support from eth1 service

[pawanjay176]

Issue Addressed

N/A

Proposed Changes

With https://github.com/sigp/lighthouse/pull/3214 we made it such that you can either have 1 auth endpoint or multiple non auth endpoints. Now that we are post merge on all networks (testnets and mainnet), we cannot progress a chain without a dedicated auth execution layer connection so there is no point in having a non-auth eth1-endpoint for syncing deposit cache.

This code removes all fallback related code in the eth1 service. We still keep the single non-auth endpoint since it's useful for testing.

Additional Info

This removes all eth1 fallback related metrics that were relevant for the monitoring service, so we might need to change the api upstream.