gluon-core: get rid of old sysctl settings

package/gluon-core/files/lib/gluon/upgrade/100-core-reset-sysctl

+#!/bin/sh
+
+# This script can be removed after Gluon v2018.2
+
+# Check for a random line that always was in /etc/sysctl.conf
+if grep -qxF 'net.ipv4.ip_forward=1' /etc/sysctl.conf; then
+	echo '# Defaults are configured in /etc/sysctl.d/* and can be customized in this file' >/etc/sysctl.conf
+fi