transcoder: Remove unused slots

Transcoder::Start() and Transcoder::Cancel() were defined as slots but
never used as such.

src/transcoder/transcoder.cpp

@@ -327,6 +327,7 @@ void Transcoder::Start() {
                    .arg(queued_jobs_.count())
                    .arg(max_threads()));
 
+  // Kick off worker threads.
   forever {
     StartJobStatus status = MaybeStartNextJob();
     if (status == AllThreadsBusy || status == NoMoreJobs) break;

src/transcoder/transcoder.h

@@ -65,7 +65,6 @@ class Transcoder : public QObject {
 
   GstPipelineModel* model() { return model_; }
 
- public slots:
   void Start();
   void Cancel();