[fix] LESS sourcemaps broken in less-plugin-clean-css v1.6.0

The ``less-plugin-clean-css`` lacks some maintenance: the sourcemaps are broken
since v1.6.0 (08/2024) [1]

- [1] https://github.com/less/less-plugin-clean-css/issues/42

Closes: https://github.com/searxng/searxng/issues/4143

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/static/themes/simple/package.json

@@ -14,7 +14,6 @@
     "grunt-xmlmin": "^0.1.8",
     "ionicons": "^7.4.0",
     "less": "^4.2.1",
-    "less-plugin-clean-css": "^1.6.0",
     "sharp": "^0.33.5",
     "stylelint": "^16.12.0",
     "stylelint-config-standard": "^36.0.0",