docs: update diagram after remove mail service

#109

README.md

@@ -21,19 +21,15 @@ flowchart LR
     api-- GET calendar -->steering[Steering API]
     api-- GET playlist -->tank[Tank API]
     ea[Engine API]
-    mail[Mailserver]
     events-- on_boot\non_sick\non_resurrect -->monitor[Monitor]
     events-- on_play\non_fallback_active -->clock[Clock]
-    events-- on_fallback_active\non_sick\non_resurrect -->mailer[Mailer]
     clock-- POST clock_data -->ea
     monitor-- PING heartbeat -->monserver[Monitoring Server]
     monitor-- POST health_data -->ea
-    mailer-- send mail -->mail
     mixer-- UNIX SOCKET -->engine_core
 
     subgraph external
         ea
-        mail
         monserver
         engine_core
         steering
@@ -61,7 +57,6 @@ flowchart LR
         subgraph plugins
             clock
             monitor
-            mailer
         end
     end
 
@@ -71,11 +66,9 @@ flowchart LR
     style tank fill:#FFF,stroke:#333,stroke-width:1px
     style monserver fill:#FFF,stroke:#333,stroke-width:1px
     style engine_core fill:#FFF,stroke:#333,stroke-width:1px
-    style mail fill:#FFF,stroke:#333,stroke-width:1px
 ```
 
 - **Event Dispatcher**: Events are issued and consumed in multiple locations of Engine. The diagram shows only a few ones, in order to support the overview.
-- **Mailer**: This module notifies on incidents and fallbacks via mail. This is not a sophisticated monitoring solution and will be [replaced](https://gitlab.servus.at/aura/engine/-/issues/109) by [Prometheus Metrics](https://gitlab.servus.at/aura/aura/-/issues/98) and a dedicated [Notification Service for Programme Coordinators](https://gitlab.servus.at/aura/aura/-/issues/139).
 - **Timetable Management**: Currently a database is used for caching and high-availability purposes. This is [subject to changes](https://gitlab.servus.at/aura/engine/-/issues/100) in subsequent releases.
 
 ## Prerequisites