KVM: x86/mmu: Drop unused CMPXCHG macro from paging_tmpl.h

Drop the CMPXCHG macro from paging_tmpl.h, it's no longer used now that
KVM uses a common uaccess helper to do 8-byte CMPXCHG.

Fixes: f122dfe4 ("KVM: x86: Use __try_cmpxchg_user() to update guest PTE A/D bits")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220613225723.2734132-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>