Revert "CMake uses FindMPI module for CMake v3.10+"
Revert "CMake uses FindMPI module for CMake v3.10+"

This reverts commit 3d28f5ccd3886201964041a7fc07ada64000cf48.

smharper/openmc,paulromano/openmc,amandalund/openmc,mit-crpg/openmc,smharper/openmc,liangjg/openmc,shikhar413/openmc,smharper/openmc,liangjg/openmc,paulromano/openmc,walshjon/openmc,amandalund/openmc,shikhar413/openmc,mit-crpg/openmc,amandalund/openmc,shikhar413/openmc,smharper/openmc,mit-crpg/openmc,paulromano/openmc,walshjon/openmc,liangjg/openmc,shikhar413/openmc,mit-crpg/openmc,liangjg/openmc,walshjon/openmc,amandalund/openmc,paulromano/openmc,walshjon/openmc