kvm: Remove the unused macro KVM_MMU_READ_{,UN}LOCK()

No code is using KVM_MMU_READ_LOCK() or KVM_MMU_READ_UNLOCK().  They
used to be in virt/kvm/pfncache.c:

                KVM_MMU_READ_LOCK(kvm);
                retry = mmu_notifier_retry_hva(kvm, mmu_seq, uhva);
                KVM_MMU_READ_UNLOCK(kvm);

However, since 58cd407c ("KVM: Fix multiple races in gfn=>pfn cache
refresh", 2022-05-25) the code is only relying on the MMU notifier's
invalidation count and sequence number.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Message-Id: <20221207120617.9409-1-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>