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/transport.go

@@ -19,50 +19,16 @@ var HTTPClient ContextKey
 // because nobody else can create a ContextKey, being unexported.
 type ContextKey struct{}
 
-// ContextClientFunc is a func which tries to return an *http.Client
-// given a Context value. If it returns an error, the search stops
-// with that error.  If it returns (nil, nil), the search continues
-// down the list of registered funcs.
-type ContextClientFunc func(context.Context) (*http.Client, error)
+var appengineClientHook func(context.Context) *http.Client
 
-var contextClientFuncs []ContextClientFunc
-
-func RegisterContextClientFunc(fn ContextClientFunc) {
-	contextClientFuncs = append(contextClientFuncs, fn)
-}
-
-func ContextClient(ctx context.Context) (*http.Client, error) {
+func ContextClient(ctx context.Context) *http.Client {
 	if ctx != nil {
 		if hc, ok := ctx.Value(HTTPClient).(*http.Client); ok {
-			return hc, nil
+			return hc
 		}
 	}
-	for _, fn := range contextClientFuncs {
-		c, err := fn(ctx)
-		if err != nil {
-			return nil, err
-		}
-		if c != nil {
-			return c, nil
-		}
+	if appengineClientHook != nil {
+		return appengineClientHook(ctx)
 	}
-	return http.DefaultClient, nil
-}
-
-func ContextTransport(ctx context.Context) http.RoundTripper {
-	hc, err := ContextClient(ctx)
-	// This is a rare error case (somebody using nil on App Engine).
-	if err != nil {
-		return ErrorTransport{err}
-	}
-	return hc.Transport
-}
-
-// ErrorTransport returns the specified error on RoundTrip.
-// This RoundTripper should be used in rare error cases where
-// error handling can be postponed to response handling time.
-type ErrorTransport struct{ Err error }
-
-func (t ErrorTransport) RoundTrip(*http.Request) (*http.Response, error) {
-	return nil, t.Err
+	return http.DefaultClient
 }