root_file_system: Remove sysctl.conf

OpenWRT changed their structure to use sysctl.d subfiles.

The default sysctl.conf is empty and kernel.panic=3 is defined
elsewhere. Thus, we do not need sysctl.conf anymore.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>