m4: Drop needless string checks
m4: Drop needless string checks

We provide default values for both MODPROBE and RMMOD and thus
there is no way that their paths can be empty strings.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

jfehlig/libvirt,jardasgit/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,eskultety/libvirt,crobinso/libvirt,zippy2/libvirt,andreabolognani/libvirt,jfehlig/libvirt,crobinso/libvirt,olafhering/libvirt,libvirt/libvirt,andreabolognani/libvirt,nertpinx/libvirt,jardasgit/libvirt,libvirt/libvirt,andreabolognani/libvirt,jardasgit/libvirt,nertpinx/libvirt,crobinso/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,andreabolognani/libvirt,olafhering/libvirt,libvirt/libvirt,jardasgit/libvirt,eskultety/libvirt,eskultety/libvirt,crobinso/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,andreabolognani/libvirt,eskultety/libvirt,zippy2/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,eskultety/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,zippy2/libvirt,nertpinx/libvirt,olafhering/libvirt