google: remove a useless build tag restriction

I missed this in CL 146677.

Change-Id: Ie2735ba15d41e51fd5a99cba97514cd16399abaa
Reviewed-by: Ross Light <>
diff --git a/google/doc.go b/google/doc.go
index 2a86325..73be629 100644
--- a/google/doc.go
+++ b/google/doc.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
-// +build go1.9
 // Package google provides support for making OAuth2 authorized and authenticated
 // HTTP requests to Google APIs. It supports the Web server flow, client-side
 // credentials, service accounts, Google Compute Engine service accounts, and Google