OpenMPI-2.*: remove --with-threads configure option.

OpenMPI builds now warn they don't know --with-threads.
See OpenMPI commit at
https://github.com/open-mpi/ompi/commit/7a55d49ca78bcc157749c04027515f12b026ec33
sed -i 's/--with-threads=posix //' OpenMPI-2*.eb