Remove unused build_fog_file from orphan artifact cleanup

What does this MR do and why

build_fog_file in ProcessList has no caller. The class has no include that could dispatch to it dynamically, and run! delegates the actual work to another class, so nothing in the file or outside it reaches the method.

Removing unused code here follows existing precedent in this repository, for example !251893 (merged), which removed an unused scope.

How to verify

Search the repository for build_fog_file. The only remaining hit is the definition being removed here.

Spotted by itaruby(to be released), a Ruby type checker I'm working on.