net/mpich2: remove
MPICH2 has been marked as deprecated since dec. 2020, and no port in the
tree depends on it anymore.
Upstream switched to MPICH3 (net/mpich) and MPICH4 (not yet in the ports
tree), and now it does not build with GCC-11: it’s time to remove it.
PR: 264366
Reported by: salvadore