DEV: Drop WithServiceHelper

This patch removes the `with_service` helper from the code base.
Instead, we can pass a block with actions directly to the `.call` method
of a service.

This simplifies how to use services:
- use `.call` without a block to run the service and get its result
  object.
- use `.call` with a block of actions to run the service and execute
  arbitrary code depending on the service outcome.

It also means a service is now “self-contained” and can be used anywhere
without having to include a helper or whatever.

app/controllers/admin/config/flags_controller.rb

@@ -2,7 +2,7 @@
 
 class Admin::Config::FlagsController < Admin::AdminController
   def toggle
-    with_service(Flags::ToggleFlag) do
+    Flags::ToggleFlag.call do
       on_success do
         Discourse.request_refresh!
         render(json: success_json)
@@ -26,7 +26,7 @@ class Admin::Config::FlagsController < Admin::AdminController
   end
 
   def create
-    with_service(Flags::CreateFlag) do
+    Flags::CreateFlag.call do
       on_success do
         Discourse.request_refresh!
         render json: result.flag, serializer: FlagSerializer, used_flag_ids: Flag.used_flag_ids
@@ -40,7 +40,7 @@ class Admin::Config::FlagsController < Admin::AdminController
   end
 
   def update
-    with_service(Flags::UpdateFlag) do
+    Flags::UpdateFlag.call do
       on_success do
         Discourse.request_refresh!
         render json: result.flag, serializer: FlagSerializer, used_flag_ids: Flag.used_flag_ids
@@ -57,7 +57,7 @@ class Admin::Config::FlagsController < Admin::AdminController
   end
 
   def reorder
-    with_service(Flags::ReorderFlag) do
+    Flags::ReorderFlag.call do
       on_success do
         Discourse.request_refresh!
         render(json: success_json)
@@ -73,7 +73,7 @@ class Admin::Config::FlagsController < Admin::AdminController
   end
 
   def destroy
-    with_service(Flags::DestroyFlag) do
+    Flags::DestroyFlag.call do
       on_success do
         Discourse.request_refresh!
         render(json: success_json)