Remove highlightjs CSS (#19483)

This was completely unnecessary: react-sdk includes appropriate CSS
for highlight.js in its themes. This was actually causing some of
those values to be overridden with silly ones that made text invisible
on the dark theme.

Fixes https://github.com/vector-im/element-web/issues/19476

package.json

@@ -58,7 +58,6 @@
     "@matrix-org/olm": "https://gitlab.matrix.org/api/v4/projects/27/packages/npm/@matrix-org/olm/-/@matrix-org/olm-3.2.3.tgz",
     "browser-request": "^0.3.3",
     "gfm.css": "^1.1.2",
-    "highlight.js": "^11.3.1",
     "jsrsasign": "^10.2.0",
     "katex": "^0.12.0",
     "matrix-js-sdk": "github:matrix-org/matrix-js-sdk#develop",