Remove sefltests' kvm_memcmp_hva_gva(), which has literally never had a
single user since it was introduced by commit 783e9e51 ("kvm:
selftests: add API testing infrastructure").

Link: https://lore.kernel.org/r/20240802200853.336512-1-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>