KVM: selftests: remove unused struct 'memslot_antagonist_args'

'memslot_antagonist_args' is unused since the original
commit f73a3446 ("KVM: selftests: Add memslot modification stress
test").

Remove it.
Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20240602235529.228204-1-linux@treblig.orgSigned-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/memslot_modification_stress_test.c

@@ -53,12 +53,6 @@ static void vcpu_worker(struct memstress_vcpu_args *vcpu_args)
 	}
 }
 
-struct memslot_antagonist_args {
-	struct kvm_vm *vm;
-	useconds_t delay;
-	uint64_t nr_modifications;
-};
-
 static void add_remove_memslot(struct kvm_vm *vm, useconds_t delay,
 			       uint64_t nr_modifications)
 {