DEV: Remove superfluous js: true metadata (#21960)

Why this change?

It is very unlikely that we need to ever JS for system tests considering
that we rely on a JS framework on the frontend.

plugins/chat/spec/system/silenced_user_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-RSpec.describe "Silenced user", type: :system, js: true do
+RSpec.describe "Silenced user", type: :system do
   fab!(:channel_1) { Fabricate(:category_channel) }
 
   let(:chat) { PageObjects::Pages::Chat.new }