chore: delete unused files (#14592)

This PR cleans up unused files in several packages within the
google-cloud-python repository.

Author: ohmayr

.librarian/generator-input/client-post-processing/bigquery-storage-integration.yaml

@@ -508,11 +508,5 @@ replacements:
           bigquery_storage_v1beta2/types_
           bigquery_storage_v1alpha/services_
           bigquery_storage_v1alpha/types_\n\n
-      Migration Guide
-      ---------------\n
-      See the guide below for instructions on migrating to the 2.x release of this library.\n
-      .. toctree::
-          :maxdepth: 2\n
-          UPGRADING\n
       Changelog
     count: 1

packages/google-cloud-bigquery-storage/UPGRADING.md

@@ -30,16 +30,6 @@ API Reference
     bigquery_storage_v1alpha/types_
 
 
-Migration Guide
----------------
-
-See the guide below for instructions on migrating to the 2.x release of this library.
-
-.. toctree::
-    :maxdepth: 2
-
-    UPGRADING
-
 Changelog
 ---------