Remove collectd network-online.target dependency

Now that *dhcpcd.service* and *dhcpcd-wait-online.service* correctly
activate *network.target* and *network-online.target*, respectively, the
hack to fix the  *collectd* *wait_prometheus* plugin binding to the TCP
socket is no longer needed.

rootfs/overlay/usr/lib/systemd/system/collectd.service.d/after-network-online.conf

@@ -1,7 +0,0 @@
-[Unit]
-# If collectd starts before eth0 is "UP," it will fail to bind to the TCP
-# socket for Prometheus:
-#
-#     write_prometheus plugin: Opening a listening socket for [::]:9103 failed.
-After=network-online.target
-Wants=network-online.target