Remove appearance task and handle unregister plugin JS

[ilyasfoo]

Changes proposed in this Pull Request:

DO NOT MERGE until https://github.com/Automattic/wc-calypso-bridge/pull/1357 is merged and Jan 2, 8pm PT

This PR:

• Removes appearance task
• Fixes cases where JS plugins are registered earlier than plugins.pluginRegistered action hook

How to test the changes in this Pull Request:

1. If https://github.com/Automattic/wc-calypso-bridge/pull/1357 is not yet merged, enable CYS feature flag:
   1. Open _cli on your Woo Express or free trial site, example https://yoursite.wpcomstaging.com/_cli
   2. Run plugin install https://gist.github.com/ilyasfoo/6c5900753edcf06faef9a4b04dca23a8/archive/0c3e3f4d3de60b84fe7e8b950ffcf753f7ec1ddf.zip --activate
2. Go to Homescreen
3. Observe Customize your store task is loaded
4. Observe other tasks are also loaded properly
5. Click on Start Customizing
6. Observe you're redirected to Customize Store screen

Other information:

• [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes, as applicable?

FOR PR REVIEWER ONLY:

• [ ] I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.

[github-actions[bot]]

Size Change: -53 B (0%)

Total Size: 196 kB

Filename	Size	Change
./build/index.js	122 kB	-53 B (0%)

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./build/53.js` | 1.05 kB | | `./build/index.css` | 883 B | | `./build/marketing.js` | 58 kB | | `./build/payment-gateway-suggestions.css` | 1.25 kB | | `./build/payment-gateway-suggestions.js` | 6.45 kB | | `./build/plugins.js` | 3.92 kB | | `./build/style-index.css` | 2.15 kB | | `./build/style-marketing.css` | 805 B |

_{compressed-size-action}

[ilyasfoo]

Merging now since CYS seems stable