internal: remove RegisterContextClientFunc

This function added a totally unused error path, since the only call
site is for App Engine, which cannot produce an error.

Change-Id: I86277ab4ff96e7bd140c53c5a114a338716668e3
Reviewed-on: https://go-review.googlesource.com/85935
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/client_appengine.go

@@ -0,0 +1,13 @@
+// Copyright 2018 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.
+
+// +build appengine
+
+package internal
+
+import "google.golang.org/appengine/urlfetch"
+
+func init() {
+	appengineClientHook = urlfetch.Client
+}