fix!: Remove google.cloud.documentai_v1beta2 (#13218)

Fixes b/371200502

Only `v1` and `v1beta3` are currently being published: 

https://github.com/googleapis/googleapis/tree/master/google/cloud/documentai

Author: parthea

packages/google-cloud-documentai/docs/documentai_v1beta2/document_understanding_service.rst

@@ -14,14 +14,6 @@ API Reference
     documentai_v1/services_
     documentai_v1/types_
 
-API Reference
--------------
-.. toctree::
-    :maxdepth: 2
-
-    documentai_v1beta2/services_
-    documentai_v1beta2/types_
-
 API Reference
 -------------
 .. toctree::