refactor: remove duplicate functionality (#314)

The `isConflict()` method duplicates the functionality of the
`IsError()` method.

hcloud/client.go

@@ -290,7 +290,7 @@ func (c *Client) Do(r *http.Request, v interface{}) (*Response, error) {
 			err = errorFromResponse(resp, body)
 			if err == nil {
 				err = fmt.Errorf("hcloud: server responded with status code %d", resp.StatusCode)
-			} else if isConflict(err) {
+			} else if IsError(err, ErrorCodeConflict) {
 				c.backoff(retries)
 				retries++
 				continue
@@ -309,14 +309,6 @@ func (c *Client) Do(r *http.Request, v interface{}) (*Response, error) {
 	}
 }
 
-func isConflict(error error) bool {
-	err, ok := error.(Error)
-	if !ok {
-		return false
-	}
-	return err.Code == ErrorCodeConflict
-}
-
 func (c *Client) backoff(retries int) {
 	time.Sleep(c.backoffFunc(retries))
 }