Remove vendor/ folder

I started to like working on projects that **don't** use `vendor/`
folders anymore. The Go ecosystem has improved a lot over the years.
With the introduction of the Go module proxy, it's more efficient and
faster for users as well.

If you're interested, here is a blog post I wrote once that goes into
more details:
https://arslan.io/2019/08/02/why-you-should-use-a-go-module-proxy/

Author: fatih

.github/workflows/go.yml

@@ -18,8 +18,7 @@ jobs:
 
     - name: Test
       run: |
-        go mod tidy -v
-        go test -mod=vendor -race ./...
+        go test -race ./...
 
     - name: Build
-      run: go build -mod=vendor ./...
+      run: go build  ./...