remove the error return value from wire.Frame.MinLength

No functional change expected.
The error was only non-nil if some required values for the STOP_WAITING
frame were not set. It should be sufficient to throw an error when
attempting to write an invalid STOP_WAITING frame.

internal/wire/ping_frame.go

@@ -28,6 +28,6 @@ func (f *PingFrame) Write(b *bytes.Buffer, version protocol.VersionNumber) error
 }
 
 // MinLength of a written frame
-func (f *PingFrame) MinLength(version protocol.VersionNumber) (protocol.ByteCount, error) {
-	return 1, nil
+func (f *PingFrame) MinLength(version protocol.VersionNumber) protocol.ByteCount {
+	return 1
 }