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/Core/IOS/DolphinDevice.cpp

@@ -138,8 +138,7 @@ IPCReply GetRealProductCode(Core::System& system, const IOCtlVRequest& request)
   if (!file.ReadBytes(data.data(), data.size()))
     return IPCReply(IPC_ENOENT);
 
-  Common::SettingsHandler gen;
-  gen.SetBytes(data);
+  Common::SettingsHandler gen(data);
   const std::string code = gen.GetValue("CODE");
 
   const size_t length = std::min<size_t>(request.io_vectors[0].size, code.length());