selftests: kvm: Remove absent target file

There is no vmx_pi_mmio_test file. Remove it to get rid of error while
creation of selftest archive:

rsync: [sender] link_stat "/kselftest/kvm/x86_64/vmx_pi_mmio_test" failed: No such file or directory (2)
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1333) [sender=3.2.3]

Fixes: 6a581508 ("selftest: KVM: Add intra host migration tests")
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Message-Id: <20220210172352.1317554-1-usama.anjum@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>