ref(gitignore): clean up old ignores, remove os-specific files

files to be ignored like .DS_Store on OSX should exist in a global
.gitignore file, where it is ignored across all repositories.

See https://help.github.com/articles/ignoring-files/#create-a-global-gitignore
I think we should add it back, too. Considering that it is something that we generate using our happy path, it should be ignored in the local gitignore.