void return, not boolean success, from some Track virtual functions...

... The return codes were mostly ignored anyway, and exceptions will be thrown
instead.

It seems there was also confusion whether the return values of Track::Paste
and Track::SyncLockAdjust were to indicate success or indicate whether there
was any change.  No matter now.

src/effects/Silence.cpp

@@ -97,7 +97,6 @@ bool EffectSilence::GenerateTrack(WaveTrack *tmp,
                                   const WaveTrack & WXUNUSED(track),
                                   int WXUNUSED(ntrack))
 {
-   bool bResult = tmp->InsertSilence(0.0, GetDuration());
-   wxASSERT(bResult);
-   return bResult;
+   tmp->InsertSilence(0.0, GetDuration());
+   return true;
 }