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);