chore(cleanup): Remove python-3.9 resources

As we move inevitably toward a Konflux-enabled build for `notebooks/` repo - I have been working on refactoring all `Dockerfile` files to remove "chained builds".  This work has been completed previously in `opendatahub-io/notebooks`.  **However**, as I look to apply the same changes to `red-hat-data-services/notebooks` - I noticed we have a **LOT** of `xxx-python-3.9` resource folders still on `main` branch.

With a desire to achieve a state of consistency in how our Dockerfiles are structured - and a lack of appetite to refactor another 10-20 `Dockerfile` files related to `python-3.9` - particularly since we have no real **need** and/or **reason** to keep this files on the `main` branch, I'm proceeding to "blow it all up".

Confirmed on slack removal of `python-3.9` resources is safe from a business perspective:
- https://redhat-internal.slack.com/archives/C060A5FJEAD/p1741364827779139

As a result, this commit removes all `xxx-python-3.9` image build directories.

A few related changes were also included:
- removed `rhel9` `python-3.9` `Makefile` target
- updated an arbitrary inline comment in `test_main.py` to reference `python-3.11`

Note: `openshift/release` configs for `red-hat-data-services` already do **NOT** reference any `python-3.9` images - so no additional/accompanying PR needs open to reflect this work
- https://github.com/openshift/release/blob/master/ci-operator/config/red-hat-data-services/notebooks/red-hat-data-services-notebooks-main.yaml

Related-to: https://issues.redhat.com/browse/RHOAIENG-19048

Author: andyatmiami

Makefile

@@ -193,11 +193,6 @@ rocm-runtime-pytorch-ubi9-python-$(RELEASE_PYTHON_VERSION):
 rocm-runtime-tensorflow-ubi9-python-$(RELEASE_PYTHON_VERSION):
 	$(call image,$@,runtimes/rocm-tensorflow/ubi9-python-$(RELEASE_PYTHON_VERSION)/Dockerfile.rocm)
 
-####################################### Buildchain for Python using RHEL9 (DEPRECATED) #######################################
-.PHONY: base-rhel9-python-3.9
-base-rhel9-python-3.9:
-	$(call image,$@,base/rhel9-python-3.9/Dockerfile)
-
 ####################################### Deployments #######################################
 
 # Download kubectl binary