Stop using methods deprecated in Wagtail

Some code was using methods from Wagtail, even though those methods were
deprecated with alternatives provided. Those alternatives are now used
instead.

wagtail/api/v2/serializers.py

@@ -8,7 +8,6 @@
 from rest_framework.fields import Field, SkipField
 from taggit.managers import _TaggableManager
 
-from wagtail.utils.compat import get_related_model
 from wagtail.wagtailcore import fields as wagtailcore_fields
 
 from .utils import get_full_url, pages_for_site
@@ -363,7 +362,7 @@ def build_relational_field(self, field_name, relation_info):
         if relation_info.to_many:
             model = getattr(self.Meta, 'model')
             child_relations = {
-                child_relation.field.rel.related_name: get_related_model(child_relation)
+                child_relation.field.rel.related_name: child_relation.related_model
                 for child_relation in get_all_child_relations(model)
             }
 

wagtail/wagtaildocs/views/multiple.py

@@ -1,11 +1,11 @@
 from django.core.exceptions import PermissionDenied
 from django.http import HttpResponseBadRequest, JsonResponse
 from django.shortcuts import get_object_or_404, render
+from django.template.loader import render_to_string
 from django.utils.encoding import force_text
 from django.views.decorators.http import require_POST
 from django.views.decorators.vary import vary_on_headers
 
-from wagtail.utils.compat import render_to_string
 from wagtail.wagtailadmin.utils import PermissionPolicyChecker
 from wagtail.wagtailsearch.backends import get_search_backends