`MarginalModel.unmarginalize` doesn't accept `var_names`

```python
import pymc as pm
from pymc_experimental import MarginalModel

with MarginalModel() as m:
    x = pm.Bernoulli("x", p=0.5)
    y = pm.Normal("y", x)
    
m.marginalize(["x"])
m.unmarginalize(["x"])  # ValueError: list.remove(x): x not in list
```

`m.unmarginalize([x])` is fine, but this is problematic because `x` may not be accessible to the caller

CC @zaxtax 