KVM: x86: Remove .get_pkru() from kvm_x86_ops

The commit

	9dd21e104bc ('KVM: x86: simplify handling of PKRU')

removed all users and providers of that call-back, but
didn't remove it. Remove it now.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>