Main: Move the G_reader_settings flush to Device:exit

I don't trust PB's input.closeAll not to be doing something wonky that
would trip an early abort.

reader.lua

@@ -292,12 +292,9 @@ end
 
 -- Exit
 local function exitReader()
-    -- Shutdown hardware abstraction
+    -- Shutdown hardware abstraction (it'll also flush G_reader_settings to disk)
     Device:exit()
 
-    -- Flush settings to disk
-    G_reader_settings:close()
-
     if Profiler then Profiler.stop() end
 
     if type(exit_code) == "number" then