Retire @azure/arm-deploymentmanager from the JS SDK monorepo (#38573)

### Packages impacted by this PR

- `@azure/arm-deploymentmanager` (removed)

### Issues associated with this PR

- Retirement request from service decommissioning discussion
(image/context in issue)
- Related spec retirement context:
Azure/azure-rest-api-specs#26818

### Describe the problem that is addressed by this PR

`@azure/arm-deploymentmanager` remained in this repo after the
underlying service/spec had already been decommissioned. This PR
completes SDK retirement by removing the package and all repository
wiring that still referenced it.

- **Package retirement**
  - Removed `sdk/deploymentmanager/arm-deploymentmanager/**` entirely.
- **Repo integration cleanup**
  - Removed `sdk/deploymentmanager/ci.mgmt.yml`.
- Removed CODEOWNERS entry for
`/sdk/deploymentmanager/arm-deploymentmanager/`.
- **Workspace metadata cleanup**
  - Removed the package importer block from `pnpm-lock.yaml`.
- **Merge conflict resolution**
- Resolved modify/delete conflicts for `package.json` and
`warp.config.yml` (kept deletions).
- Resolved `pnpm-lock.yaml` conflict by incorporating upstream changes
while retaining removal of the `arm-deploymentmanager` importer block.

### What are the possible designs available to address the problem? If
there are more than one possible design, why was the one in this PR
chosen?

- **Design A: keep package, mark deprecated**  
Leaves dead CI/ownership/lockfile surface and prolongs maintenance of a
retired SDK.
- **Design B: full retirement (chosen)**  
Removes the package and all references in one change set, matching prior
monorepo retirement pattern and eliminating stale maintenance footprint.

### Are there test cases added in this PR? _(If not, why?)_

No. This is a package retirement/removal change; there is no remaining
implementation surface in this repo for new tests.

### Provide a list of related PRs _(if any)_

- Retirement pattern reference:
#37675

### Checklists
- [ ] Added impacted package name to the issue description.
- [ ] Does this PR need any fixes in the SDK Generator?** _(If so,
create an Issue in the
[Autorest/typescript](https://github.com/Azure/autorest.typescript)
repository and link it here.)_
- [ ] Added a changelog (if necessary).

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jeremymeng <7583839+jeremymeng@users.noreply.github.com>
Co-authored-by: kazrael2119 <98569699+kazrael2119@users.noreply.github.com>

Author: 3 people

.github/CODEOWNERS

@@ -535,9 +535,6 @@
 # PRLabel: %Mgmt
 /sdk/dataprotection/arm-dataprotection/ @qiaozha @MaryGao @JialinHuang803
 
-# PRLabel: %Mgmt
-/sdk/deploymentmanager/arm-deploymentmanager/ @qiaozha @MaryGao @JialinHuang803
-
 # PRLabel: %Mgmt
 /sdk/dependencymap/arm-dependencymap/ @qiaozha @MaryGao @JialinHuang803