djvu_desktop: wait for spawned Au_.exe process when removing

what was wrong with using the /S?

The /S is still there but that just makes the install silent. The problem I was having was that, on remove, wpkg would tell me that the software was still present despite having been uninstalled when I manually checked. That usually means that the remove cmd has finished running before wpkg finishes and/or that the uninstaller spawns a child process. And that's what was happening here so I added a check to wait for the spawned process.

I wasn't seeing the problem of WPKG telling me the software was still installed hence why I sent a merge request. To me it was installing and uninstalling correctly, maybe because I'm running on new PC.