Remove lightningcss (#29070)

Remove lightningcss and minify with esbuild again. The size of output
CSS will increase by around 1.4%, but I think it's worth it to allow
building gitea in more cases like the one in the linked issue. We can
reconsider once lightningcss becomes more stable.

Fixes: https://github.com/go-gitea/gitea/issues/29058

package.json

@@ -29,7 +29,6 @@
     "jquery": "3.7.1",
     "katex": "0.16.9",
     "license-checker-webpack-plugin": "0.2.1",
-    "lightningcss-loader": "2.1.0",
     "mermaid": "10.7.0",
     "mini-css-extract-plugin": "2.8.0",
     "minimatch": "9.0.3",