Remove Transport.CancelRequest log message

[andig]

Follow-up to #400: I've just encountered

deprecated: golang.org/x/oauth2: Transport.CancelRequest no longer does anything; use contexts

in the log and asked on the ML: https://groups.google.com/g/golang-nuts/c/-hheka0vc40 since it seems that this message cannot be avoided with the default http.Client:

// For compatibility, the Client will also use the deprecated
// CancelRequest method on Transport if found. New
// RoundTripper implementations should use the Request's Context
// for cancellation instead of implementing CancelRequest.
Timeout time.Duration

While the client is convert by the compatibility promise, oauth2 is imho not.

I'd like to propose to remove the deprecated CancelRequest method.