Beautify minigeth diff

[norswap]

This issue collects a couple of improvements to be made to the minigeth diff:

1. Deleting stale minigeth files

When rebasing minigeth on top l2geth, I did not pay particular attention to new files that might have been introduced but are unused.

There might not be a lot of those, because I had to remove some because they depended on things that had been removed previously.

2. Remove odd comments, misc cleanups

As listed in the Minigeth Diff wiki page (see recap at the bottom).

[norswap]

Another suggestion: StateDB#ForEachStorage doesn't work — we could comment out the interface definition and the implementation to make sure it doesn't get called inadvertently after a rebase.

[protolambda]

Minigeth is no longer used by Cannon.