fix: remove JWT_DECODE_HANDLER override

[robrap]

Remove the override of JWT_DECODE_HANDLER in the settings files, because it already matches the default set in base.py, and when we wish to remove this custom
jwt_decode_handler, it is possible that this setting will unnecessarily cause an error in the future. See
https://github.com/openedx/ecommerce/blob/b5fa2e55e3d4f5f8224d381317177cf4086b35a6/ecommerce/settings/base.py#L449

The JWT_AUTH dictionary should not be replaced as a whole, but should only replace keys that are declared, according to this:
https://github.com/openedx/ecommerce/blob/b5fa2e55e3d4f5f8224d381317177cf4086b35a6/ecommerce/settings/production.py#L51-L53

Note: This change is very similar to https://github.com/edx/edx-internal/pull/8163 which already landed.

Configuration Pull Request

Make sure that the following steps are done before merging:

• A SRE team member has approved the PR if it is code shared across multiple services and you don't own all of the services.
• Are you adding any new default values that need to be overridden when this change goes live? If so:
  • Update the appropriate internal repo (be sure to update for all our environments)
  • If you are updating a secure value rather than an internal one, file a SRE ticket with details.
  • Add an entry to the CHANGELOG.
• If you are making a complicated change, have you performed the proper testing specified on the Ops Ansible Testing Checklist? Adding a new variable does not require the full list (although testing on a sandbox is a great idea to ensure it links with your downstream code changes).
• Think about how this change will affect Open edX operators. Have you updated the wiki page for the next Open edX release?

[pshiu]

Thanks, @robrap!

[pshiu]

Note: We are having a temporary issue with the Jenkins job that builds the ecommerce Docker image for devstack.

Until this PR can be incorporated into devstack via that Jenkins job, developers experiencing issues like:

ImportError: Could not import 'ecommerce.extensions.api.handlers.jwt_decode_handler' for API setting 'JWT_DECODE_HANDLER'. ModuleNotFoundError: No module named 'ecommerce.extensions.api.handlers'.

after bringing up a new container of the ecommerce devstack can make it work by temporarily adding:

'JWT_DECODE_HANDLER': 'edx_rest_framework_extensions.auth.jwt.decoder.jwt_decode_handler',

to this dictionary on their local. (Apologies for the inconvenience.)

[robrap]

@pshiu: Will this PR temporarily fix the issue for everyone without special instructions? openedx/ecommerce#3958. I did not test.

[pshiu]

Great idea, thanks @robrap! Just gave a thumb and added an AC to edx/upgrades#109.

[robrap]

Great @pshiu. As I noted, this is untested, so just make sure it resolves your issue. Thanks.

[pshiu]

@robrap, yep, @jsnwesson and I had tested an almost identical patch right before I saw your comment.