webp: remove Go 1.8 support

This is a revert of CL 24945 and CL 24954, since we no longer need to
support Go 1.5.

No need to explicitly specify which Go version is required. Instead,
rely on the implicit requirement based on the release support policy¹.
This is simpler and consistent with how it was before CL 24954.

¹ https://tip.golang.org/doc/devel/release.html#policy

Change-Id: I3cfef1bef120f702d5859dad10a76803544cc869
Reviewed-on: https://go-review.googlesource.com/c/148569
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/webp/decode.go b/webp/decode.go
index 111f358..f77a4eb 100644
--- a/webp/decode.go
+++ b/webp/decode.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 go1.6
-
 package webp
 
 import (
diff --git a/webp/decode_test.go b/webp/decode_test.go
index b27468a..ad65b10 100644
--- a/webp/decode_test.go
+++ b/webp/decode_test.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 go1.6
-
 package webp
 
 import (
diff --git a/webp/doc.go b/webp/doc.go
new file mode 100644
index 0000000..e321c85
--- /dev/null
+++ b/webp/doc.go
@@ -0,0 +1,9 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package webp implements a decoder for WEBP images.
+//
+// WEBP is defined at:
+// https://developers.google.com/speed/webp/docs/riff_container
+package webp // import "golang.org/x/image/webp"
diff --git a/webp/webp.go b/webp/webp.go
deleted file mode 100644
index 850cdc8..0000000
--- a/webp/webp.go
+++ /dev/null
@@ -1,30 +0,0 @@
-// Copyright 2016 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// Package webp implements a decoder for WEBP images.
-//
-// WEBP is defined at:
-// https://developers.google.com/speed/webp/docs/riff_container
-//
-// It requires Go 1.6 or later.
-package webp // import "golang.org/x/image/webp"
-
-// This blank Go file, other than the package clause, exists so that this
-// package can be built for Go 1.5 and earlier. (The other files in this
-// package are all marked "+build go1.6" for the NYCbCrA types introduced in Go
-// 1.6). There is no functionality in a blank package, but some image
-// manipulation programs might still underscore import this package for the
-// side effect of registering the WEBP format with the standard library's
-// image.RegisterFormat and image.Decode functions. For example, that program
-// might contain:
-//
-//	// Underscore imports to register some formats for image.Decode.
-//	import _ "image/gif"
-//	import _ "image/jpeg"
-//	import _ "image/png"
-//	import _ "golang.org/x/image/webp"
-//
-// Such a program will still compile for Go 1.5 (due to this placeholder Go
-// file). It will simply not be able to recognize and decode WEBP (but still
-// handle GIF, JPEG and PNG).