Remove pointless storage of var names in virNWFilterHashTable
    
    The virNWFilterHashTable struct contains a virHashTable and
    then a 'char **names' field which keeps a copy of all the
    hash keys. Presumably this was intended to record the ordering
    of the hash keys. No code ever uses this and the ordering is
    mangled whenever a variable is removed from the hash, because
    the last element in the list is copied into the middle of the
    list when shrinking the array.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>