Remove interop genesis service from beacon node

[prestonvanloon]

What type of PR is this?

Other

What does this PR do? Why is it needed?

The intention of this PR is to reduce unnecessary dependencies in the beacon node and reduce legacy tech debt. The features of --interop-genesis-time and --interop-num-validators can be replaced by generating the beacon state with external tooling and using it within the beacon node via --genesis-state=/path/to/genesis.ssz.

Running a devnet_

Example workflow prior to this PR:

# Setup an EL... <omitted>
# And then, start Prysm with a genesis time 30s into the future and 1024 validators. Other flags
omitted.
./beacon-chain \
  --interop-genesis-time=$(date -d "30 seconds" +%s) \
  --interop-num-validators=1024 \
 <other flags omitted>

The new workflow is an additional step, but common practice with devnets.

# Setup an EL... <omitted>
# And then, create the genesis.ssz with a start time 30s into the future.
./prysmctl testnet generate-genesis \
  --num-validators=1024 \
  --genesis-time-delay=30 \
  --output-ssz=/data/genesis.ssz

./beacon-chain \
  --genesis-state=/data/genesis.ssz \ # Use the generated genesis state.
  <other flags omitted>

Which issues(s) does this PR fix?

Other notes for review

Note: The interop functionality of the validator is unchanged for now. The flags --interop-num-validators and --interop-start-index remain in the validator client until a follow up PR.

Acknowledgements

• I have read CONTRIBUTING.md.
• I have made an appropriate entry to CHANGELOG.md.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.