[flake8-pyi] Add autofix for unused-private-type-var (PYI018)

[ntBre]

We currently offer a diagnostic for unused private type variables like these:

import typing
import typing_extensions

_T = typing.TypeVar("_T")
_Ts = typing_extensions.TypeVarTuple("_Ts")

But it would be nice to offer a fix to delete them too. This came up in #15682 because a PYI018 autofix would finish the transformation of pre-PEP-695 code like this:

from typing import Generic, TypeVar

_T = TypeVar("_T")

class Foo(Generic[_T]):
    var: _T

to

class Foo[T]:
    var: T

by converting from Generic + standalone TypeVar to PEP-695 generics (UP046, renaming the private generic (UP051), and then removing the now-unused private type variable (PYI018 after this issue).

The function implementing this rule can be found here. I think delete_stmt may help with the implementation, with some examples of its usage here and here.