remove sometimes unused variable `old`

The argument of `PRIO2INDEX` is only used in the parallel version of
UG; building the sequential version would raise a warning about `old`
being unused.

This change eliminates the `old` variables.

This addresses the following warning from gcc-7:

    dune-uggrid/gm/ugio.cc:1736:33: warning:
    variable ‘old’ set but not used [-Wunused-but-set-variable]