Remove old patterns for legacy Go versions (pre 1.0, when Makefiles were used instead of go command).

It hurts me to see those remnants over and over again.

Remove vendor/.

To quote https://blog.gopheracademy.com/advent-2015/vendor-folder/:

Advice on how to use the vendor folder is varied. Some will shutter at the thought of including dependencies in the project repository. Others hold it is unthinkable to not include the dependencies in the project repository. Some will just want to include a manifest or lock file and fetch the dependencies before building.

Go doesn't have widely used gem and rubygems.org equivalent yet. Instead, several third-party tools are used, which fetch dependencies from Github and other places, and lock versions to particular commit hashes. go tool is then used to build programs. go is aware of vendor/ directory, but knows nothing about lock files, commit hashes, etc. If vendor/ is not included in the repository, then go get will download sources – but it will be the latest commit for each import path (more or less is equivalent of repository URL), not specified commits from lock file. In you are lucky, and all package authors do care about backward compatibility (but that's not true in general – many packages are broken regularly), it may work. If you are less lucky, you will get a broken build. In the worst case, your code will explode in runtime, not during compilation. Therefore, I'm strongly for inclusion of vendor/. I think the majority of the community thinks the same way.

Even people who disagree with this opinion would agree that omitting vendor/ should be opt-in, not opt-out.

Related pull requests:
#2198 (closed as duplicate).
#2183 (closed due to conflicts).