Drop halt_callback_chains_on_return_false

ActiveSupport.halt_callback_chains_on_return_false= is deprecated and
will be removed in Rails 5.2. Stop using it and return `throw(:abort)`
instead in the callbacks which need it.

app/models/role.rb

@@ -18,6 +18,6 @@ class Role < ApplicationRecord
 
   # Needed to ensure that removing all user from role doesn't remove role.
   def cancel
-    false
+    throw(:abort)
   end
 end