Change: Remove vendored dependencies

We've removed the vendored dependencies (in the subdir `vendor/`). When
building restic, the Go compiler automatically fetches the dependencies. It
will also cryptographically verify that the correct code has been fetched by
using the hashes in `go.sum` (see the link to the documentation below).

https://github.com/restic/restic/issues/2482
https://golang.org/cmd/go/#hdr-Module_downloading_and_verification
