Drop verify single RpcBlock fns

[dapplion]

Issue Addressed

Noted that we never have to verify individual RpcBlocks for sync. Lookup sync sends the block alone always, and range sync sends chain segments.

Proposed Changes

• Removed verify_kzg_for_rpc_block
• Removed IntoExecutionPendingBlock impl for RpcBlock. Now lookup sync blocks use the SignedBeaconBlock IntoExecutionPendingBlock impl which is the same
• Switched tests to import RpcBlocks with process_chain_segment