Fix incorrect (and useless) conversion of a wxString to and from std::string.

This useless conversion breaks non ASCII8 strings.

common/settings/settings_manager.cpp

@@ -339,7 +339,7 @@ wxString SETTINGS_MANAGER::GetPathForSettingsFile( JSON_SETTINGS* aSettings )
         return GetUserSettingsPath();
 
     case SETTINGS_LOC::PROJECT:
-        return std::string( Prj().GetProjectPath() );
+        return Prj().GetProjectPath();
 
     case SETTINGS_LOC::COLORS:
         return GetColorSettingsPath();