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 #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": "15.0.0-rc.1",

src/vector/index.ts

@@ -22,7 +22,6 @@ limitations under the License.
 // Our own CSS (which is themed) is imported via separate webpack entry points
 // in webpack.config.js
 require('gfm.css/gfm.css');
-require('highlight.js/styles/github.css');
 require('katex/dist/katex.css');
 
 /**