tox: The remove func for contacts now takes a list

This replaces neuland_tox_remove_contact(), which could only remove one
contact at a time, with neuland_tox_remove_contacts(), which takes a
GSList of contacts.
tox: Fix void function signature
tox: Accept contacts func now takes a list

This makes it more flexible and easier to use with a more than one
selected contact once the selection mode will be functional.