[amber] Revert "[amber] use the configured transport with oauth2"

This reverts commit 9d839ff7d804ce5702c271d46d7cca9a66fc84a8. The oauth
wrapper uses the provided transport anyway, and replacing it disables
the oauth token injection.

Test: manual

Change-Id: I56d0dd1841b434aad9ca5df9b9d4e904c58c4300
diff --git a/go/src/amber/source/source.go b/go/src/amber/source/source.go
index dc6edec..5e72c11 100644
--- a/go/src/amber/source/source.go
+++ b/go/src/amber/source/source.go
@@ -365,13 +365,11 @@
 	ctx := context.WithValue(context.Background(), oauth2.HTTPClient, httpClient)
 
 	timeout := httpClient.Timeout
-	transport := httpClient.Transport
 
 	tokenSource = cfg.TokenSource(ctx, token)
 	httpClient = oauth2.NewClient(ctx, tokenSource)
 
 	httpClient.Timeout = timeout
-	httpClient.Transport = transport
 
 	return httpClient, tokenSource
 }