Chore(appengine) Remove deprecated samples (#13833)

* Remove deprecated samples
Removal of appengine/flexible_python37_and_earlier path for deprecation.
As per my search on Code Search, there are no samples using these deprecated code snippets anymore.
b/475332253

* Remove reference to python 3.7 earlier samples from README.md file

Author: XrossFox

appengine/flexible/README.md

@@ -7,8 +7,6 @@
 
 These are samples for using Python on Google App Engine Flexible Environment. These samples are typically referenced from the [docs](https://cloud.google.com/appengine/docs).
 
-For code samples of Python version 3.7 and earlier, please check
-https://github.com/GoogleCloudPlatform/python-docs-samples/tree/main/appengine/flexible_python37_and_earlier
 
 See our other [Google Cloud Platform github repos](https://github.com/GoogleCloudPlatform) for sample applications and
 scaffolding for other frameworks and use cases.