Use or remove commented function `GetPubKey` in `script/sign.cpp`
As condition of merging !723 (merged), I'm opening this issue to schedule the future complete removal -- or use -- of the commented out function.
!723 (merged) is a special case in that it really is planned that the function will be used, and it's generating a nuisance compiler warning right now which bothers many (including myself). Removing it completely will make the backporting of those commits which use it slightly harder
In this case we make a short tradeoff between leaving commented code in for a brief time. In case we don't use it within a month, we should remove it.