Fix C4189 and C4100 Warnings

C4189 is 'Local variable initialised but not used'
C4100 is 'Unreferenced parameter'

Fixed some other warnings, e.g. about empty if, too.

src/commands/ScriptCommandRelay.cpp

@@ -107,7 +107,6 @@ int ExecCommand2(wxString *pIn, wxString *pOut)
       CommandBuilder builder(*pIn);
       if (builder.WasValid())
       {
-         AudacityProject *project = GetActiveProject();
          OldStyleCommandPointer cmd = builder.GetCommand();
          AppCommandEvent ev;
          ev.SetCommand(cmd);

src/commands/SelectCommand.cpp

@@ -230,6 +230,7 @@ bool SelectTracksCommand::Apply(const CommandContext &context)
       // Add 0.01 so we are free of rounding errors in comparisons.
       constexpr double fudge = 0.01;
       for (auto channel : channels) {
+         (void)channel;// compiler food
          double track = index + fudge + term;
          bool sel = first <= track && track <= last;
          if( mMode == 0 ){ // Set