package/mosquitto: extend systemd integration

package/mosquitto: extend systemd integration

Since version 1.5, mosquitto can be built with explicit support for
systemd. If enabled, libmosquitto will link against libsystemd: when
started, the mosquitto broker notifies systemd that it is ready (ie.
initialized and ready to accept connections), so that services that
depend on the mqtt broker can be started only at that point.

To enable this feature, the systemd service config file needs to change
to Type=notify. Upstream now provides such a file, so we can remove
ours.

Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>