Fix Rollup warnings and remove some unused Rollup plugins (#26387)

* Fix rollup warnings

* Remove some unused Rollup plugins

* Re-add unintentionally half-removed Stylelint

webapp/channels/package.json

@@ -95,6 +95,7 @@
     "smooth-scroll-into-view-if-needed": "1.1.33",
     "stream-browserify": "3.0.0",
     "styled-components": "5.3.6",
+    "stylelint": "13.13.1",
     "timezones.json": "1.6.1",
     "tinycolor2": "1.4.2",
     "turndown": "7.1.1",
@@ -173,7 +174,6 @@
     "react-router-enzyme-context": "1.2.0",
     "redux-mock-store": "1.5.4",
     "redux-persist-node-storage": "2.0.0",
-    "stylelint": "13.13.1",
     "stylelint-config-idiomatic-order": "8.1.0",
     "stylelint-config-recommended": "5.0.0",
     "stylelint-config-recommended-scss": "4.2.0",