docs: Try harder to uninstall

docs: Try harder to uninstall

Imagine you have partially installed libvirt, or maybe you're
just running 'make uninstall' from a different version than 'make
install' has been ran. One way or another, we are doing plain
'rm' instead of 'rm -f' and thus not trying hard enough when
uninstalling. In the rest of our code we stick with -f switch. Do
that for docs too.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>