remove drop_dups index option, deprecated with MongoDB3

docs/changelog.rst

@@ -10,8 +10,10 @@ Development
 - Fixed a bug causing inaccurate query results, while combining ``__raw__`` and regular filters for the same field #2264
 - Add support for the `elemMatch` projection operator in .fields() (e.g BlogPost.objects.fields(elemMatch__comments="test")) #2267
 - DictField validate failed without default connection (bug introduced in 0.19.0) #2239
-- Remove name parameter in Field constructor e.g `StringField(name="...")`, it was deprecated a while ago in favor of db_field
-- Remove method queryset.slave_okay() that was deprecated a while ago and disappeared since pymongo3
+- Remove methods deprecated years ago:
+    - name parameter in Field constructor e.g `StringField(name="...")`, was replaced by db_field
+    - Queryset.slave_okay() was deprecated since pymongo3
+    - dropDups was dropped with MongoDB3
 
 Changes in 0.19.1
 =================

docs/guide/defining-documents.rst

@@ -555,7 +555,6 @@ There are a few top level defaults for all indexes that can be set::
             'index_background': True,
             'index_cls': False,
             'auto_create_index': True,
-            'index_drop_dups': True,
         }
 
 
@@ -574,11 +573,6 @@ There are a few top level defaults for all indexes that can be set::
     in systems where indexes are managed separately. Disabling this will improve
     performance.
 
-:attr:`index_drop_dups` (Optional)
-    Set the default value for if an index should drop duplicates
-    Since MongoDB 3.0 drop_dups is not supported anymore. Raises a Warning
-    and has no effect
-
 
 Compound Indexes and Indexing sub documents
 -------------------------------------------