docs: remove *other* reference to igmp for IPv6
docs: remove *other* reference to igmp for IPv6

This finishes the job started by commit 81f3839f8 - removing the
erroneous reference to nonexistent "igmp-ipv6" protocol.

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