Remove needless uses of wxString::c_str() in wxString::Format...

... and similar wx "variadics," which all treat wxString smartly enough that
you don't need this.

Don't need c_str either to convert wxString to const wxChar * because
wxString has a conversion operator that does the same.

src/prefs/KeyConfigPrefs.cpp

@@ -622,9 +622,9 @@ void KeyConfigPrefs::OnSet(wxCommandEvent & WXUNUSED(event))
       if (AudacityMessageBox(
             wxString::Format(
             _("The keyboard shortcut '%s' is already assigned to:\n\n\t'%s'\n\nClick OK to assign the shortcut to\n\n\t'%s'\n\ninstead.  Otherwise, click Cancel."),
-            key.c_str(),
-            oldlabel.c_str(),
-            newlabel.c_str()),
+            key,
+            oldlabel,
+            newlabel),
             _("Error"), wxOK | wxCANCEL | wxICON_STOP | wxCENTRE, this) == wxCANCEL)
       {
          return;