Remove stray uses of "golang.org/x/net/context"

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/build_cancel.go

@@ -1,9 +1,8 @@
 package client // import "github.com/docker/docker/client"
 
 import (
+	"context"
 	"net/url"
-
-	"golang.org/x/net/context"
 )
 
 // BuildCancel requests the daemon to cancel ongoing build request

client/hijack_test.go

@@ -1,6 +1,7 @@
 package client
 
 import (
+	"context"
 	"fmt"
 	"io/ioutil"
 	"net"
@@ -12,7 +13,6 @@ import (
 	"github.com/docker/docker/api/server/httputils"
 	"github.com/docker/docker/api/types"
 	"github.com/pkg/errors"
-	"golang.org/x/net/context"
 	"gotest.tools/assert"
 )