[mlir][Python] remove stray nb::cast (#176299)

In https://github.com/llvm/llvm-project/pull/155114 we removed
`liveOperations` but forgot this line which was being used to invalidate
operations under a transform root, which currently isn't being used for
anything. So remove.

FYI this led to a subtle double free bug after
https://github.com/llvm/llvm-project/pull/175405:

```python
@test_in_context
def check_builtin():
    module = builtin_d.ModuleOp()
    with module.context, ir.Location.unknown():
        transform_module = builtin_d.Module.create()
        transform_module.operation.attributes["transform.with_named_sequence"] = (
            ir.UnitAttr.get()
        )
        with ir.InsertionPoint(transform_module.body):
            named_sequence = NamedSequenceOp("__transform_main", [any_op_t()], [])
            with ir.InsertionPoint(named_sequence.body):
                YieldOp([])
        interp.apply_named_sequence(
            module,
            transform_module.body.operations[0],
            transform_module,
        )
```

with error 

```
python(7436,0x1f95a93c0) malloc: *** error for object 0x6000002b0000: pointer being freed was not allocated
python(7436,0x1f95a93c0) malloc: *** set a breakpoint in malloc_error_break to debug
```

This is because

```
nb::object obj = nb::cast(payloadRoot);
```
is actually equivalent to
```
nb::object obj = nb::cast(payloadRoot, nb::rv_policy::automatic);
```
which is actually equivalent to
```
nb::object obj = nb::cast(payloadRoot, nb::rv_policy::copy);
```
because I changed the API to `PyOperationBase &payloadRoot` i.e., an
lvalue reference and `nb::rv_policy::automatic` decays to
`nb::rv_policy::copy` for [lvalue
refs](https://nanobind.readthedocs.io/en/latest/api_core.html#_CPPv4N8nanobind9rv_policy9automaticE).