models: Remove explicit id fields.

With django-stubs, these explicit copies of Django’s implicit id
fields are no longer needed for type checking.  An exception is the
BigAutoField AbstractUserMessage.id, which is left alone.

This reverts commit c08ee904d8 (#15641).

Signed-off-by: Anders Kaseorg <anders@zulip.com>

corporate/models.py

@@ -345,7 +345,6 @@ class LicenseLedger(models.Model):
 
 
 class ZulipSponsorshipRequest(models.Model):
-    id = models.AutoField(auto_created=True, primary_key=True, verbose_name="ID")
     realm = models.ForeignKey(Realm, on_delete=CASCADE)
     requested_by = models.ForeignKey(UserProfile, on_delete=CASCADE)