[#210] remove extensions from __init__.py

These should be imported wherever necessary in the API's themselfs

vng_api_common/inspectors/__init__.py

@@ -1,10 +1 @@
-from .fields import (
-    GegevensGroepExtension,
-    HyperlinkedIdentityFieldExtension,
-    HyperlinkedRelatedFieldExtension,
-    ReadOnlyFieldExtension,
-)
-from .files import FileFieldExtension
-from .geojson import GeometryFieldExtension
-from .polymorphic import PolymorphicSerializerExtension
-from .query import FilterExtension
+