Remove dead rc.emhttp mapping from adapter.sh

No such rc.d service exists on Unraid (emhttpd isn't managed via a
standalone rc.emhttp script) and nothing in the codebase calls
platform_restart_service/platform_stop_service with 'emhttp' — confirmed
during a post-7.3.2-upgrade audit of all platform-specific paths.

Plugin/unraid/adapter.sh

@@ -61,7 +61,6 @@ _platform_rc_script() {
         rsyslog) echo "/etc/rc.d/rc.rsyslogd" ;;
         nginx)   echo "/etc/rc.d/rc.nginx" ;;
         php-fpm) echo "/etc/rc.d/rc.php-fpm" ;;
-        emhttp)  echo "/etc/rc.d/rc.emhttp" ;;
         *)       echo "/etc/rc.d/rc.$1" ;;
     esac
 }