pcplusmp shouldn't support x2APIC mode
We are currently in the weird situation to have two platform modules dealing with APIC and x2APIC, namely pcplusmp and apix. Initially there was only pcplusmp that handled the APIC. Then support for x2APIC was added to pcplusmp. A little later a new module apix was introduced to deal with systems supporting x2APIC in both legacy APIC and x2APIC modes. Ever since then all systems supporting x2APIC were using apix, not pcplusmp.
The two modules share a fair bit of code, which isn't surprising as both support the legacy APIC mode. But there really is no reason for pcplusmp to have the ability to work in x2APIC mode. It's relatively easy to garbage-collect the x2APIC support from pcplusmp by moving a few pieces of code around with no functional changes.