Remove the unused file parsing function

With the new xt_bpf support for iface stats. We no longer need to parse
the per interface stats from /proc/net/dev. And since the old xt_qtaguid
code path also not depend on it, we can completly remove that helper
function since no caller is depending on it now.

Bug: 72111305
Test: runtest frameworks-net -c com.android.internal.net.NetworkStatsFactoryTest
Change-Id: Icb7eaeef0eeb9fdffd32a90316c76ee05bafffbe