Avoid wallet code writing node settings file

Change wallet loading code to access settings through the Chain
interface instead of writing settings.json directly.

src/util/system.h

@@ -207,6 +207,7 @@ protected:
      */
     bool UseDefaultSection(const std::string& arg) const EXCLUSIVE_LOCKS_REQUIRED(cs_args);
 
+ public:
     /**
      * Get setting value.
      *
@@ -221,7 +222,6 @@ protected:
      */
     std::vector<util::SettingsValue> GetSettingsList(const std::string& arg) const;
 
-public:
     ArgsManager();
     ~ArgsManager();