chore: delete owlbot.py

[parthea]

Now that googleapis/synthtool#1244 is merged, owlbot.py is no longer required in this repo. We can add owlbot.py back in the future if repository specific customizations are needed.

[parthea]

TODO: Determine if the changes in this CL would be considered a breaking change as the reference documentation moved from https://github.com/googleapis/python-iam/tree/main/docs/credentials_v1 to https://github.com/googleapis/python-iam/tree/main/docs/iam_credentials_v1

[parthea]

Note to self: if this is not a breaking change, send a CL to update the BUILD.bazel file here with the following changes:

--- a/google/iam/credentials/v1/BUILD.bazel
+++ b/google/iam/credentials/v1/BUILD.bazel
@@ -150,6 +150,11 @@ py_gapic_library(
     name = "credentials_py_gapic",
     srcs = [":credentials_proto"],
     grpc_service_config = "iamcredentials_grpc_service_config.json",
+    opt_args = [
+        "warehouse-package-name=google-cloud-iam",
+        "python-gapic-namespace=google.cloud",
+        "python-gapic-name=iam_credentials",
+    ],
 )

[parthea]

Wait for cl/408017201

[parthea]

Superseded by #136