Reset code to empty in rollback

[gumb0]

I think we don't need to save the old code for rollback anymore, because overwriting the code is not allowed according to address collision rules (ethereum/EIPs#684 and https://eips.ethereum.org/EIPS/eip-689) and we never get to rolling back the overwritten code.

A bit of history of this code:
It was written as a fix of incorrect rollback of code overwrite in case of address collision (back then it was possible to overwrite the code)
#4130
#4151

Then in preparation for Byzantium new collision rules were introduced, which applied retroactively to all previous forks, too.
#4384

To quote EIP-689:

Regarding testing, this EIP relieves clients from supporting reversion of code overwriting.

This will simplify a little bit the implementation of EIP-1702 Generalized Account Versioning Scheme, because if we don't need to save the old code in journal, then we don't need to save the version either.

[codecov-io]

Codecov Report

Merging #5641 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5641      +/-   ##
==========================================
+ Coverage   62.74%   62.76%   +0.01%     
==========================================
  Files         348      348              
  Lines       29670    29682      +12     
  Branches     3343     3343              
==========================================
+ Hits        18617    18629      +12     
- Misses       9841     9842       +1     
+ Partials     1212     1211       -1

[chfast]

Why is the EIP-689 still a draft?

[gumb0]

@chfast Probably no one cared to accept it yet. There was some discussion recently on All Core Devs call, where @holiman mentioned that it's already implemented, see discussion of "Action 60.6" in https://github.com/ethereum/pm/blob/master/All%20Core%20Devs%20Meetings/Meeting%2061.md

[halfalicious]

@gumb0 : When would we need to rollback to old code before EIP-689? When there's a chain reorg?

[gumb0]

@halfalicious No, rollback happens any time after exception during transaction execution (for example Out-of-gas or invalid instruction)