Fix some 'declaration hides' warnings.

We were for example getting many of these:
\audacity\src\widgets\numerictextctrl.h(171): warning C4458: declaration of 'value' hides class member

MSVC2013 didn't warn about these, but MSVC2017 does.

src/prefs/MidiIOPrefs.cpp

@@ -207,8 +207,6 @@ void MidiIOPrefs::OnHost(wxCommandEvent & WXUNUSED(e))
          wxString device = wxString::Format(wxT("%s: %s"),
                                             interf,
                                             name);
-         int index;
-
          if (info->output) {
             playnames.Add(name);
             index = mPlay->Append(name, (void *) info);