Don't prompt to delete SCANsat's settings at upgrade

[HebaruSan]

Problem

1. Install SCANsat v20.1
2. Use it; it will create GameData/SCANsat/PluginData/Settings.cfg
3. Upgrade to v20.2
4. You'll be asked whether to remove the PluginData folder, even though the mod is still installed
   

Cause

SCANsat creates PluginData as an empty folder and only uses it for saving its Settings.cfg, so it naturally meets the criteria for a removable config-only dir at uninstall. When we upgrade, we:

1. Remove the older version, which flags PluginData as potentially removable
2. Install the newer version, which creates the PluginData folder if it doesn't already exist but leaves it in the list of possibly removable dirs

Changes

Now when we install a directory, we remove it from the list of empty directories to ask the user about. This removes the prompt from the upgrade flow but still presents it at uninstall.