DEV: removes default service actions (#26078)

Previously services would let you define a high level default `def default_actions_for_service; end` which would define various handlers like `on_success`, after months of usage we consider the cons are superior to the pros here.

Two mains cons:
- people would often not understand where the handling was coming from
- it's easy to miss a case when you write your specs

plugins/chat/lib/service_runner.rb

@@ -27,8 +27,6 @@
 # argument to their block. `on_model_errors` receives the actual model so it’s
 # easier to inspect it.
 #
-# Default actions for each of these are defined in [Chat::ApiController#default_actions_for_service]
-#
 # @example In a controller
 #   def create
 #     with_service MyService do