remove lms lettuce test in packer builds

[estute]

Configuration Pull Request

Make sure that the following steps are done before merging:

• [ ] A DevOps 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 DEVOPS 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).

---

lms/djangoapps/courseware/features/problems.feature was removed from master recently, causing the build-packer-ami jobs to fail. There are only a few lettuce tests left in the platform, but until we are fully lettuce-free, we should still make sure that we can run it at least one.