x86, apic: Remove support for IBM Summit/EXA chipset

There should no longer be any IBM x440 systems or those using the
Summit/EXA chipset out in the wild, so remove support for it.

We've done our due diligence in reaching out to any contact information
listed for this chipset and no indication was given that it should be
kept around.

Signed-off-by: David Rientjes <rientjes@google.com>