CLOUDP-364847: Remove unused MDB_IMAGE_TYPE environment variable

[Julien-Ben]

Summary

The MDB_IMAGE_TYPE environment variable was originally used by the community operator but became orphaned after the MEKO/MCO merge. The community operator now uses MDB_COMMUNITY_IMAGE_TYPE instead. It was used by the enterprise operator until around October 2024.
Commit removing its usage: https://github.com/10gen/ops-manager-kubernetes/commit/e0127188c835c2bb374488533950824180faf490
And then, used by the community operator only.

This PR removes the unused variable from:

• Helm chart values and templates
• Operator deployment configs
• Dev and E2E test scripts
• Go code (unused constant)

As a follow up of this ticket, I created WRITING-34891, to come up with an engineering proposal, which will propose a proper solution for configurable UBI base image selection.

Note: I added the documentation changes required in the Jira ticket, for public doc.
For example we mention it here: https://www.mongodb.com/docs/kubernetes/current/reference/kubectl-operator-settings/

Proof of Work

• Builds successfully and unit tests pass.
• Public manifests regenerated via make precommit

Checklist

• Have you linked a jira ticket and/or is the ticket in the title?
• Have you checked whether your jira ticket required DOCSP changes?
• Have you added changelog file?
  • use skip-changelog label if not needed
  • refer to Changelog files and Release Notes section in CONTRIBUTING.md for more details

[github-actions]

⚠️ (this preview might not be accurate if the PR is not rebased on current master branch)

MCK 1.6.1 Release Notes

Bug Fixes

• Backed up the agent password in a secret for SCRAM authentication to prevent unnecessary password rotations.
• MongoDB Adding missing ownerrefs to ensure proper resource deletion by kubernetes.
• Single Cluster Deleting resources created by CRD now only happens on multi-cluster deployments. Single Cluster will solely rely on ownerrefs.
• MongoDB, MongoDBOpsManager: Improve validation for featureCompatibilityVersion field in MongoDB and MongoDBOpsManager spec.
  The field now enforces proper semantic versioning. Previously, invalid semver values could be accepted,
  potentially resulting in incorrect configurations.
• Roles configured via Ops Manager UI or API will no longer be removed by the operator

Other Changes

• Future releases will include a new asset, release_info_.json, which provides detailed information about each MongoDB Controllers for Kubernetes release, including a clear list of all container images. This will help customers, especially those running in air-gapped environments, easily identify all required images for a given release.
• Operator configuration: Removed the unused MDB_IMAGE_TYPE environment variable and the corresponding mongodb.imageType Helm value. This variable was deprecated in v1.28.0 of the MongoDB Enterprise Kubernetes Operator when it switched to architecture-based image selection (ubi9 for static, ubi8 for non-static). This is a cleanup change with no functional impact.

[Julien-Ben]

This is a reference architecture file

[Julien-Ben]

@vinilage I will merge once I have your approval for the release notes

[vinilage]

LGTM!