v6.x: Remove internal hwloc and libevent

[jsquyres]

Per discussions leading up to the v5.0.x release: hwloc and libevent are fairly globally available on platforms that Open MPI cares about:

1. They're both inbox in many modern Linuxes
2. They're both available via Homebrew/MacPorts for MacOS

As such, for Open MPI v6.x (i.e., the next major series after v5.x -- we don't want to do this during the v5.x series), we should remove the internal / bundled versions of hwloc and libevent.