Remove files that are already included in WordPress Core

[donnapep]

Resolves https://github.com/Automattic/sensei-security/issues/17. (Cherry-picked from https://github.com/Automattic/sensei-security/pull/28.)

Proposed Changes

• Removes the polyfill for the core-pattern block that was needed for WP 5.8. I checked WP 5.9 and confirmed that the block is registered there.
• Deletes the assets/dist/css/jquery-ui.js file after webpack creates it. As we are enqueuing the jquery-ui.css file in PHP, rather than importing it via a JS file, Webpack generates a corresponding jquery-ui.js file that is essentially empty. That is true of all CSS files that we handle this way, and is a well-documented problem. I tried using this Webpack plugin but it didn't work, so instead opted for a quick and dirty solution and just delete this particular file after Webpack does its thing.

Testing Instructions

• Create a new course, add a title and a description in the wizard, and close the modal.
• Make sure the default course template continues working.
• Create a new course, and navigate to the last step.
• Make sure you see a default course pattern.
• Choose it, and make sure it works properly.
• Run npm run build.
• Unzip the built file.
• Check that there is no assets/dist/css/jquery-ui.js file.
• Install the Plugin Check plugin.
• Go to Tools > Plugin Check.
• Select Sensei LMS from the dropdown and ensure Plugin Repo is selected.
• Click Check it! button.
• Search the page for "Library files that are already in the WordPress core are not permitted." and ensure no instances are found.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• Code is tested on the minimum supported PHP and WordPress versions

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[donnapep]

Merging as this was already approved in https://github.com/Automattic/sensei-security/pull/28.