Don't try to set the value of fm_screen_mode

I'm not sure what the original intention was, but in practice, the user
experience regarding the way this setting is saved is horrible: it seems
to change randomly and once the orientation is changed from portrait to
landscape, the only way to fix it is to edit `settings.reader.lua`.

frontend/apps/filemanager/filemanager.lua

@@ -330,7 +330,6 @@ end
 
 function FileManager:onClose()
     DEBUG("close filemanager")
-    G_reader_settings:saveSetting("fm_screen_mode", Screen:getScreenMode())
     UIManager:close(self)
     if self.onExit then
         self:onExit()