feat: Remove workaround for django.cache Datadog service tag in edxapp

[timmc-edx]

This should no longer be necessary as of Datadog 2.19.1 (tested in devstack).

---

Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.