cleanup: delete orphaned xmodule/js/src/html/display.js and imageModal.js

Context

As part of the Slash-n-Burn Built-In XBlocks epic:

• PR #38746 removes the Annotatable block, which deletes the webpack entry AnnotatableBlockDisplay — the entry that previously included xmodule/js/src/html/display.js alongside the annotatable display JS.
• PR #38748 removes the HTML block, which deletes the webpack entry HtmlBlockDisplay — the other entry that included xmodule/js/src/html/display.js and xmodule/js/src/html/imageModal.js.

After both PRs merge, neither of these files is referenced by any webpack entry:

• xmodule/js/src/html/display.js
• xmodule/js/src/html/imageModal.js

They cannot be deleted in PR #38748 before PR #38746 merges (they are still referenced by AnnotatableBlockDisplay in webpack.builtinblocks.config.js on master), so this follow-up issue tracks the cleanup.

Work to do

Once both PR #38746 and PR #38748 are merged into master:

1. Delete xmodule/js/src/html/display.js
2. Delete xmodule/js/src/html/imageModal.js
3. Remove the draggabilly.js imports-loader rule from webpack.common.config.js (line ~317) if no other entry references xmodule/js/common_static/js/vendor/draggabilly.js

This is a small standalone PR — should take ~5 minutes.

Wait until PRs #38746 and #38748 are merged, then inspect xmodule/js/src/html/display.js, xmodule/js/src/html/imageModal.js, and webpack.common.config.js. Confirm no webpack entry references these files or xmodule/js/common_static/js/vendor/draggabilly.js, delete the two orphaned files, and remove the draggabilly.js imports-loader rule if unused.