KVM: async_pf: drop kvm_arch_async_page_present wrappers

The wrappers make it less clear that the position of the call
to kvm_arch_async_page_present depends on the architecture, and
that only one of the two call sites will actually be active.
Remove them.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>