Upgrade Migration.cdc contract to complete removal of account format v1

[j1010001]

Account format was optimized by onflow/cadence#3584.

All accounts on both Testnet and mainnet networks were migrated via on-the-fly migration, we can now safely remove the support for the old (v1) account format.

[joshuahannan]

@j1010001 Does this still need to be open?