Remove image backends config from runtests.py

It is not needed since Pillow is the default and get_backend works with
full package name.

runtests.py

@@ -18,17 +18,6 @@ if not settings.configured:
     except ImportError:
         has_elasticsearch = False
 
-    WAGTAILIMAGES_BACKENDS = {
-        'default': {
-            'BACKEND': 'wagtail.wagtailimages.backends.pillow_backend.PillowBackend',
-        },
-        'pillow': {
-            'BACKEND': 'wagtail.wagtailimages.backends.pillow_backend.PillowBackend',
-        },
-        'wand': {
-            'BACKEND': 'wagtail.wagtailimages.backends.wand_backend.WandBackend',
-        },
-    }    
     WAGTAILSEARCH_BACKENDS = {
         'default': {
             'BACKEND': 'wagtail.wagtailsearch.backends.db.DBSearch',
@@ -97,7 +86,6 @@ if not settings.configured:
             'django.contrib.auth.hashers.MD5PasswordHasher',  # don't use the intentionally slow default password hasher
         ),
         WAGTAILSEARCH_BACKENDS=WAGTAILSEARCH_BACKENDS,
-        WAGTAILIMAGES_BACKENDS=WAGTAILIMAGES_BACKENDS,
         WAGTAIL_SITE_NAME='Test Site'
     )