installer: revert "installer: force uninstall before install."
installer: revert "installer: force uninstall before install."

This reverts commit 3d4663465ea1e528a299f1939237c31a4188775c.

That commit was done because of reports that stale files from
old versions of Mumble were left behind. In particular, in
the "Versions" subdirectory.

I just tested this locally, and I can't reproduce that problem.

Reverting this commit should hopefully resolve a lot issues
such as:

mumble-voip/mumble#2901
mumble-voip/mumble#1917
mumble-voip/mumble#1845
mumble-voip/mumble#1700

Perhaps more.

Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble