When we ran the migration rehearsal, the OVM_ETH migration took approximately an hour and a half. This was because the storage slots in the OVM_ETH contract were iterated twice: once to check them, and another time to actually migrate them. To reduce the amount of time taken, this PR updates the OVM_ETH migration to check and migrate in one step. This does not cause database corruption on failure, since the trie changes are buffered in memory and aren't committed until all the checks pass. Total migration time should be reduced by approximately 40 minutes.

Separately, I removed the invalid check slots referring to the EIP-1967 slots on the L2XDM. These slots no longer exist, and were causing the post-check steps to fail.

Closes CLI-3590
Closes CLI-3576