fix!: remove google.cloud.asset_v1p4beta1 (#14571)

Remove `google.cloud.asset_v1p4beta1` which doesn't exist in
https://github.com/googleapis/googleapis/tree/master/google/cloud/asset

Author: parthea

packages/google-cloud-asset/docs/asset_v1p4beta1/asset_service.rst

@@ -30,14 +30,6 @@ API Reference
     asset_v1p2beta1/services_
     asset_v1p2beta1/types_
 
-API Reference
--------------
-.. toctree::
-    :maxdepth: 2
-
-    asset_v1p4beta1/services_
-    asset_v1p4beta1/types_
-
 API Reference
 -------------
 .. toctree::