installer: directly stop box code

the idea (previously) was that the box code knew how to stop itself.
this is why stop.sh of the _old_ code was invoked. we can just inline
the code needed to stop the old version into installer.sh itself.

scripts/installer.sh

@@ -190,7 +190,7 @@ done
 
 if [[ "${is_update}" == "yes" ]]; then
     log "stop box service for update"
-    ${box_src_dir}/setup/stop.sh
+    systemctl stop box
 fi
 
 # ensure we are not inside the source directory, which we will remove now