all: remove x/net/context in favour of context

Change-Id: Idbfe40e7cb9f99ca3dc7d4c214de6b9e028eefb7
Reviewed-on: https://go-review.googlesource.com/c/148277
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

semaphore/semaphore.go

@@ -7,12 +7,8 @@ package semaphore // import "golang.org/x/sync/semaphore"
 
 import (
 	"container/list"
+	"context"
 	"sync"
-
-	// Use the old context because packages that depend on this one
-	// (e.g. cloud.google.com/go/...) must run on Go 1.6.
-	// TODO(jba): update to "context" when possible.
-	"golang.org/x/net/context"
 )
 
 type waiter struct {