Remove unused SiteMemberBackend and its model

The SiteMemberBackend and the model UserSiteMapping were added in 2016
but never used. In Tahoe we always use UserOrganizationMapping and its
backend.

The two backends are sort-of redaudant and adds maintainance cost even
if its very small.

organizations/backends.py

@@ -70,25 +70,3 @@ def authenticate(self, *args, **kwargs):  # pylint: disable=arguments-differ
             if site_organization in user_organizations:
                 return user
         return None
-
-
-class SiteMemberBackend(ModelBackend):
-    """
-    Extension of the regular ModelBackend that will check whether the user is a member of the currently
-    active site.
-
-    Deprecated: Should be removed because `UserSiteMapping` should be removed.
-    """
-
-    def authenticate(self, *args, **kwargs):  # pylint: disable=arguments-differ
-        """
-        Authenticate organization learners.
-        """
-        user = super(SiteMemberBackend, self).authenticate(*args, **kwargs)
-        # superuser can log into any microsite
-        site = get_current_site()
-        is_default_site = site.id == settings.SITE_ID
-        if not is_default_site and is_site_configuration_enabled() and user and not user.is_superuser:
-            if user.id in site.usersitemapping_set.select_related('user').values_list('user__id', flat=True):
-                return user
-        return None

organizations/migrations/0012_delete_usersitemapping.py

@@ -0,0 +1,16 @@
+# Generated by Django 2.2.12 on 2021-06-02 06:38
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('organizations', '0011_historicalorganization_edx_uuid'),
+    ]
+
+    operations = [
+        migrations.DeleteModel(
+            name='UserSiteMapping',
+        ),
+    ]

organizations/models.py

@@ -121,16 +121,3 @@ def __str__(self):
             email=self.user.email,  # pylint: disable=no-member
             organization=org.short_name,
         )
-
-
-class UserSiteMapping(models.Model):
-    """
-    User membership in a site.
-
-    Deprecated: Not used by Tahoe. Should be removed.
-    """
-
-    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
-    site = models.ForeignKey(Site, on_delete=models.CASCADE)
-    is_active = models.BooleanField(default=True)
-    is_amc_admin = models.BooleanField(default=False)