toybox: Do not install a symlink for traceroute6

toybox: Do not install a symlink for traceroute6

Installing a symlink for traceroute6 can be harmful on debug builds,
because the a race condition in the build process might lead to
external/iputils overwriting the toybox binary.
A similar issue (with the blkid toy) was fixed in 85bdd234fffc
("toybox: Fix race condition during install")

Google's toybox installation code does not install a symlink for
traceroute6 either. Debug builds are getting traceroute6 from
external/iputils instead.

FIXES: 26c5170b565 "Add new toys in Android makefile and generate links
automatically"

Change-Id: I5ef4e437d32740f08c72403aa7710452a044184a