Remove merge transition code

[dapplion]

Summary

Remove merge transition code that is no longer needed. The merge (Ethereum's transition from proof-of-work to proof-of-stake) occurred in September 2022. All related transition logic, readiness checks, and terminal difficulty validation can now be safely removed.

This PR represents the squashed and rebased version of the work at commit ca6a1b66791e29b9ab47b6faa67424ede0281a06.

Changes

This PR removes approximately 2,100 net lines of code across 39 files, including:

• Terminal difficulty detection: Removed get_terminal_pow_block_hash(), get_pow_block_at_total_difficulty(), and related terminal block hash validation logic from execution layer
• Bellatrix readiness checks: Simplified bellatrix_readiness.rs from merge transition readiness validation to just genesis execution payload consistency checks
• Optimistic transition block verification: Deleted entire otb_verification_service.rs (369 lines) and related optimistic import logic specific to the merge transition
• Merge-specific notifications: Removed terminal block found logging, merge readiness warnings, and transition status messages from notifier
• Operation pool: Cleaned up merge-specific attestation and block proposal logic that handled transition edge cases
• Test infrastructure: Removed merge transition test cases from bellatrix.rs (212 lines), capella.rs (156 lines), and other integration tests

Files Changed

Key modifications:

• beacon_node/beacon_chain/src/otb_verification_service.rs - deleted
• beacon_node/beacon_chain/src/bellatrix_readiness.rs - simplified to genesis checks only
• beacon_node/execution_layer/src/lib.rs - removed terminal difficulty methods
• beacon_node/client/src/notifier.rs - removed merge readiness logging
• beacon_node/operation_pool/src/lib.rs - cleaned up transition-specific logic
• Various test files - removed obsolete merge transition tests

Testing

• Existing test suite passes
• Removed tests were specific to merge transition scenarios that are no longer relevant
• Core functionality (block processing, attestations, execution layer integration) unchanged

[michaelsproul]

Have we tested syncing through the merge transition on e.g. mainnet?