CLEANUP: vars: Remove void vars_unset_by_name(const char*, size_t, struct sample*)

CLEANUP: vars: Remove void vars_unset_by_name(const char*, size_t, struct sample*)

With "MINOR: lua: Use vars_unset_by_name_ifexist()" the last user was
removed and as outlined in that commit there is no good reason for this
function to exist.

May be backported together with the commit mentioned above.