HiSLIP: disable heuristic dissector by default

It is too weak (just checking that the 2 first bytes of TCP segment are "HS") to be always activated

Change-Id: Iffda96046cc687004f403dd4d8e735375790b07b
Reviewed-on: https://code.wireshark.org/review/5789
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>