Update EIP-7702: add instruction to remove code at end of tx

[NiloCK]

Original version of the EIP had:

At the end of the transaction, set the contract_code of each signer back to empty.

But currently no similar instruction is present. This PR replaces it.

[eth-bot]

File EIPS/eip-7702.md

Requires 1 more reviewers from @adietrichs, @lightclient, @SamWilsn, @vbuterin

[lightclient]

This is specifically no longer a goal of 7702

[NiloCK]

This is specifically no longer a goal of 7702

Interesting, thanks.

In that case, I expect the point

Once this is implemented, allowing EOAs to migrate permanently is "only one line of code": just add a flag to not set the code back to empty at the end.

should be removed from the Forward-compatibility section.

[NiloCK]

Closing - opened #8871.