Don't do all the fd.o desktop integration stuff on OS X.

It doesn't need it, so don't install those files, or uninstall them, or
suggest running various unnecessary commands after the installation
finishes.

Change-Id: Ieecab22884723670f47f2fc7cc1ac827f48c1967
Reviewed-on: https://code.wireshark.org/review/5614
Reviewed-by: Guy Harris <guy@alum.mit.edu>