KVM: Drop unused kvm_dirty_gfn_invalid()

Drop the unused function as reported by test bot.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210901230904.15164-1-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>