ref(.gitignore): collapse distributed patterns into root .gitignore

This change removes multiple `.gitignore` files and puts the patterns which are still applicable into the root `.gitignore`. 

Arguably this makes version control exclusions easier to manage by centralizing them, but my main motivation here is my personal workflow. The atom editor misapplies some `.gitignore` patterns that reference a binary of the same name as the directory, such as `deisctl/deisctl` and `bumpver/bumpver`. The directory is hidden from tree navigation and from "Find in Project". Moving all the patterns to the root `.gitignore` works around atom's bug and lets me stop context-switching to `git grep` every 2 minutes.