Change: Remove vendored dependencies

We've removed the vendored dependencies (in the subdir `vendor/`) similar to
what we did for `restic` itself. 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).

Building the rest-server now requires Go 1.11 or newer, since we're using Go
Modules for dependency management. Older Go versions are not supported any more.

https://github.com/restic/rest-server/issues/102
https://golang.org/cmd/go/#hdr-Module_downloading_and_verification