Add missing async job controller for corerp and delete dead code. (#6243

)

# Description

* Add deployment processor for corerp resource deletion
* Delete dead code in corerp

## Type of change

- This pull request is a minor refactor, code cleanup, test improvement,
or other maintenance task and doesn't change the functionality of Radius
(issue link optional).

## Auto-generated summary

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GitHub Copilot for docs will auto-generate a summary of the PR
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### <samp>🤖 Generated by Copilot at 0c13b30</samp>

### Summary
🗑️🛣️📦

<!--
1. 🗑️ - This emoji represents the deletion or removal of something,
which is the main purpose of the added operations.
2. 🛣️ - This emoji represents the HTTPRoute resource type, which is used
to define how requests are routed to different services or endpoints in
Radius.
3. 📦 - This emoji represents the ContainerResource and VolumeResource
resource types, which are used to define the containers and volumes that
run the applications and store the data in Radius.
-->
This pull request implements delete functionality for custom resources
in the Radius core package. It also removes two unused files from the
`pkg/corerp/backend` directory.

> _Oh, we're the crew of the Radius ship, and we sail the code sea_
> _We create and update our resources, but sometimes we must delete_
> _So we pull on the rope and we sing this song, as we heave on the
count of three_
> _`HTTPRoute`, `ContainerResource`, `Gateway`, and `VolumeResource`,
goodbye to ye_

### Walkthrough
* Add delete operations for custom resources
([link](https://github.com/radius-project/radius/pull/6243/files?diff=unified&w=0#diff-1dfecb9519c9002f569daea7fb61bbb0b1f22e34d49ac2d03eeb3bc64e858c8fR92-R94),
[link](https://github.com/radius-project/radius/pull/6243/files?diff=unified&w=0#diff-1dfecb9519c9002f569daea7fb61bbb0b1f22e34d49ac2d03eeb3bc64e858c8fR115-R117),
[link](https://github.com/radius-project/radius/pull/6243/files?diff=unified&w=0#diff-1dfecb9519c9002f569daea7fb61bbb0b1f22e34d49ac2d03eeb3bc64e858c8fR138-R140),
[link](https://github.com/radius-project/radius/pull/6243/files?diff=unified&w=0#diff-1dfecb9519c9002f569daea7fb61bbb0b1f22e34d49ac2d03eeb3bc64e858c8fR161-R163))
* Remove unused files
([link](https://github.com/radius-project/radius/pull/6243/files?diff=unified&w=0#diff-5d6b976ca1401aa4b4ea6fdaf8142700ada0d6ddd9690bb7eed8797f91d33466),
[link](https://github.com/radius-project/radius/pull/6243/files?diff=unified&w=0#diff-0fb3b9f2c9139180171c66e076eacdb9f6ea1b44e8a3356a8dbc688436614535))