teamviewer: fix brokenness after updates / GC

Teamviewer puts symlinks to nix store paths in ~/.teamviewer. When those
paths become garbage collected, teamviewer crashes upon start because of
those broken symlinks. An easy workaround to this behaviour is simply to
delete all symlinks before we start teamviewer. Teamviewer will fixup
the symlinks, just like it did the first time the user ran it.
Merge branch 'teamviewer-fix-upgrades' of git://github.com/bjornfor/nixpkgs

teamviewer: fix brokenness after updates / GC