Revert "We only need one rule for mu and one rule for nu in muGV"

This reverts commit 18d19cfaf24814209a50064db9743c6e37a5f631.

All existing work on session types treats mu G and G (mu G) as
equivalent.  Not treating them equivalently deviates from previous work,
and so requires explanation while providing no particular benefit.
There is no term in muGV to implement the eliminated typing rule, as
such a term would require access to map.