Remove backported --warning-color var from onionNetError.css once we are esr92-based

We've back-ported the css logic for setting --warning-color for onion-service related error icons. It is specified in aboutNetError.css ( https://searchfox.org/mozilla-central/source/browser/themes/shared/aboutNetError.css#7 ) in esr92. See this bug for details:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1721658