Remove is_classmethod_class slot from CallableType (#16151)

This slot was not used anywhere:

```
» ag is_classmethod_class .
                           
```

Moreover, since it was not initialized this code was failing with
`AttributeError`:

```python
x: CallableType
for i in dir(x):
    print(i, getattr(x, i))  # failing on `is_classmethod_class`
```

mypy/types.py

@@ -1763,8 +1763,6 @@ class CallableType(FunctionLike):
         "definition",  # For error messages.  May be None.
         "variables",  # Type variables for a generic function
         "is_ellipsis_args",  # Is this Callable[..., t] (with literal '...')?
-        "is_classmethod_class",  # Is this callable constructed for the benefit
-        # of a classmethod's 'cls' argument?
         "implicit",  # Was this type implicitly generated instead of explicitly
         # specified by the user?
         "special_sig",  # Non-None for signatures that require special handling