Use unified block production from updated eth2 client

[nkryuchkov]

Since https://github.com/attestantio/go-eth2-client/pull/116 GetBeaconBlock can return both proposal and blinded proposal.

This PR

• removes the logic which eth2 client endpoint to choose
• always uses GetBeaconBlock instead

TODO:

• tests

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 21.73913% with 36 lines in your changes are missing coverage. Please review.

Project coverage is 49.0%. Comparing base (86cddd7) to head (623609b).

Files	Patch %	Lines
beacon/goclient/proposer.go	0.0%	21 Missing :warning:
ekm/eth_key_manager_signer.go	17.6%	14 Missing :warning:
cli/operator/node.go	0.0%	1 Missing :warning:

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[moshe-blox]

would rather we use official spec tag but I think its best we don't wait for it.

yeah we need to start testing this