traceroute6: remove -l flag

The -l flag was used to tell traceroute6(8) to show both hostname and
address for each hop. However, traceroute(8) already does this by
default, and there's no reason for traceroute6 to behave differently.

Make this the default behaviour, and accept -l for backward
compatibility as a no-op flag.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1023