chore(managed-identities): rm unused config API path (#12856)

Removing unused API path from config for managed-identities, so that it
matches the generated code status.

v1beta1 of java-managed-identities was added to the
generation_config.yaml as part of initial migration to hermetic build:
14b7346,
but client library code for v1beta1 is not present in
java-managed-identities/google-cloud-managed-identities dir. This is due
to deep-copy-regex being too strict and does not copy /v1beta1 from
staging dir to final output.

Expect generation workflow to remove sample code related to v1beta1.

For #12736
For googleapis/librarian#5370

---------

Co-authored-by: cloud-java-bot <cloud-java-bot@google.com>

Author: zhumin8

generation_config.yaml

@@ -1558,7 +1558,6 @@ libraries:
   api_id: managedidentities.googleapis.com
   GAPICs:
   - proto_path: google/cloud/managedidentities/v1
-  - proto_path: google/cloud/managedidentities/v1beta1
 - api_shortname: managedkafka
   name_pretty: Managed Service for Apache Kafka
   product_documentation: https://cloud.google.com/managed-kafka