device: drop rp_filter handling

After commit b1082aa9 (device: disable
rp_filter handling) drop the now unused code.

https://bugzilla.redhat.com/show_bug.cgi?id=1651097
5 jobs for th/sysctl-and-rp_filter in 19 minutes and 15 seconds (queued for 13 seconds)
Status Job ID Name Coverage
  Test
passed #62806
checkpatch

00:01:29

passed #62807
f28_build_autotools

00:06:56

passed #62808
f28_build_meson

00:04:28

passed #62809
f28_rpm_autotools_dbg

00:07:07

passed #62810
f28_rpm_meson_dbg

00:06:20