Windows installer 8.5.x+ must actively remove legacy plugins folder

[michaelherger]

As seen in https://forums.slimdevices.com/forum/user-forums/general-discussion/1689778-lms-windows-64bit-logs-where-are-they?p=1690363#post1690363 an update would only overwrite existing files, but not remove files no longer available in newer versions. We have to add a few lines to the installer to get rid of those defunct plugin folders.