Eliminate SettingsHandler's SetBytes and Reset methods

Also make the `Decrypt` method private.

As far as I can tell, the only motivation for exposing the `SetBytes`
and `Reset` methods is to allow `CBoot::SetupWiiMemory` to use the same
`SettingsHandler` instance to read settings data and then write it back.
It seems cleaner to just use two separate instances, and require a given
`SettingsHandler` instance to be used for either writing data to a
buffer or reading data from a buffer, but not both.

A natural next step is to split the `SettingsHandler` class into two
classes, one for writing data and one for reading data. I've deferred
that change for a future PR.

Source/Core/Common/SettingsHandler.h

@@ -30,14 +30,12 @@ public:
   void AddSetting(std::string_view key, std::string_view value);
 
   const Buffer& GetBytes() const;
-  void SetBytes(const Buffer& buffer);
   std::string GetValue(std::string_view key) const;
 
-  void Decrypt();
-  void Reset();
   static std::string GenerateSerialNumber();
 
 private:
+  void Decrypt();
   void WriteLine(std::string_view str);
   void WriteByte(u8 b);