oauth2: remove direct dependency on golang.org/x/net

Change-Id: If8fd952f4bfd3bac3e85592a49616adf4b43ce51
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/460764
Reviewed-by: Cody Oss <codyoss@google.com>
Run-TryBot: Matt Hickford <matt.hickford@gmail.com>
Reviewed-by: Matt Hickford <matt.hickford@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

internal/token.go

@@ -19,8 +19,6 @@ import (
 	"strings"
 	"sync"
 	"time"
-
-	"golang.org/x/net/context/ctxhttp"
 )
 
 // Token represents the credentials used to authorize
@@ -229,7 +227,7 @@ func RetrieveToken(ctx context.Context, clientID, clientSecret, tokenURL string,
 }
 
 func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
-	r, err := ctxhttp.Do(ctx, ContextClient(ctx), req)
+	r, err := ContextClient(ctx).Do(req.WithContext(ctx))
 	if err != nil {
 		return nil, err
 	}