Do not substitute strdup().

Do not substitute strdup().

libpcap has been using strdup() for a long time too, but never
substituted it under the assumption the function is available in all
supported OSes, see libpcap commit cb71eef from 2006.  The only
exception to that is Windows, which can have a different name for the
function, but both libpcap and tcpdump for that already use a separate
workaround, which does not involve the substitution.

Let's take this as a proof that strdup() substitution in tcpdump is dead
code, and remove it.