Made runtests.py run gis_tests only when using a GIS database backend.

This facilitates other changes like refs #28160.

tests/gis_tests/relatedapp/models.py

@@ -4,7 +4,6 @@ from django.contrib.gis.db import models
 class SimpleModel(models.Model):
     class Meta:
         abstract = True
-        required_db_features = ['gis_enabled']
 
 
 class Location(SimpleModel):

tests/gis_tests/relatedapp/tests.py

@@ -12,7 +12,6 @@ from .models import (
 )
 
 
-@skipUnlessDBFeature("gis_enabled")
 class RelatedGeoModelTest(TestCase):
     fixtures = ['initial']