Remove root key env var dead code

[Neon-White]

Describe the Problem

In continuation to noobaa/noobaa-operator#1533 (which also contains a more elaborate explanation), this PR removes the dead code paths that are no longer relevant due to the removal of the NOOBAA_ROOT_SECRET environment variable

Explain the Changes

1. load_root_key() was removed since it only performed an env var loading
2. system_store now calls load_root_keys_from_mount() instead of load_root_key() to load the root key from the mount point instead of the env var
3. test_encryption no longer tests load_root_key() since it was removed
4. get_root_key_id() no longer handles cases where the key might be in the env
5. Removed the env var from the NVA build as well

Issues:

Fixed:

1. DFBUGS-1608

Testing Instructions:

1. Deploy NooBaa
2. Perform some day 1 operations (bucket creation, bucket IO)

• Doc added/updated
• Tests added

[Copilot]

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/test/unit_tests/test_encryption.js:906

• Verify that the removal of the subsequent load calls (previously used to reload the system state) does not leave the active root key state unvalidated. Consider adding explicit assertions to ensure that the state update is properly reflected after modifying the active root key file.

await fs.promises.writeFile(config.ROOT_KEY_MOUNT + '/active_root_key', 'key1');

src/server/system_services/master_key_manager.js:52

• Confirm that the revised get_root_key_id behavior (which now solely relies on this.active_root_key) correctly handles cases when active_root_key is undefined, since the environment variable fallback was removed.

return this.active_root_key && this.active_root_key.toString();