Don't complain when deleting non-existent tethering rules.

clearTetheringRules ignores errors when deleting rules,
because tethering rules don't exist unless tethering was
enabled on the interface. sendNetlinkRequest shouldn't log an
error in this case, since the caller is ignoring that error.

Bug: 34873832
Test: bullhead builds, boots, spurious error messages gone
Change-Id: Ib327e8a3aecd3a38d624baa8bf320da87e6c4f7c