cleanup - remove tun.h

Longer term we should probably just merge the clat_config
and tun_data structs, but not just yet.

Test:
  git grep '(^|[^_])tun[.]h'
  comes up empty

Bug: 144730808
Test: atest clatd_test netd_integration_test
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I7c3a986584aa3d02519545fb928457b61247ed14
7 files changed