[skip-ci] docs: note removal of Kafka producer from FTP door

Assisted-by: deepseek-v4-flash/desy-assistant

Author: dCache Bot

docs/TheBook/src/main/markdown/kafkaproducer.md

@@ -54,12 +54,15 @@ dcache.kafka.topic = billing
 "billing" is default. The following service level variables
 reference dcache.kafka.topic:
 
-   ftp.kafka.topic = ${dcache.kafka.topic}
    nfs.kafka.topic = ${dcache.kafka.topic}
    pool.kafka.topic = ${dcache.kafka.topic}
    webdav.kafka.topic = ${dcache.kafka.topic}
    xrootd.kafka.topic = ${dcache.kafka.topic}
 
+Note that the FTP door no longer supports pushing messages to Kafka; the
+producer for FTP has been removed and any `ftp.kafka.*` properties are now
+obsolete.
+
 ### 3. Start Server
 
 ```console-root