ZBX_JAVAGATEWAY_ENABLE is no longer handled by any entrypoint since the
images were migrated to native Zabbix environment variables. The Java
gateway is now configured directly via ZBX_JAVAGATEWAY (JavaGateway) and
ZBX_STARTJAVAPOLLERS (StartJavaPollers).
The obsolete variable was still documented in the server/proxy READMEs and
shipped in the example env files and kubernetes.yaml, where it does nothing.
Worse, the README described it as the way to enable the Java gateway, so a
user setting only ZBX_JAVAGATEWAY_ENABLE=true would get no Java checks.
Remove all remaining references (READMEs, env_vars/.env_srv, env_vars/.env_prx,
kubernetes.yaml). The native ZBX_JAVAGATEWAY / ZBX_STARTJAVAPOLLERS variables
are kept, so existing setups keep working.