Use write interceptor from faas-provider

We now have two write interceptors, with one moved into
faas-provider. This commit makes the gateway use the new
external package and deletes its own.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alex@openfaas.com>

gateway/vendor/github.com/openfaas/faas-provider/httputil/writers.go

@@ -0,0 +1,12 @@
+package httputil
+
+import (
+	"fmt"
+	"net/http"
+)
+
+// Errorf sets the response status code and write formats the provided message as the
+// response body
+func Errorf(w http.ResponseWriter, statusCode int, msg string, args ...interface{}) {
+	http.Error(w, fmt.Sprintf(msg, args...), statusCode)
+}