governance: remove migration from `GovernanceDelegation`

[cairoeth]

Description & Context

For interop, the governor will have to be updated use the L1 block number instead of the OP L2 block number as networks in their superchain will have their own cadence of blocks.

Proposed by @smartcontracts, the interop update introduces breaking changes so it's better to have the delegation state separated: the legacy state in the GovernanceToken (with L2 block number), and the new state in the GovernanceDelegation contract.