remove non-model overload for Field descriptor

[asottile-sentry]

(not upstreamed)

hopefully eventually can remove when python/mypy#5570 is solved

little testcase for this:

from __future__ import annotations

from django.db import models

class M1(models.Model):
    f: models.Field[int, int] = models.IntegerField()

class C:
    f: int

def f(inst: C | M1 | M2) -> int:
    return inst.f

# should also work with field subclasses
class F(models.Field[int, int]):
    pass

class M2(models.Model):
    f = F()

def g(inst: C | M2) -> int:
    return inst.f