Remove the optional strncasecmp.{h,c} target (for systems that don't have that API).

strncasecmp() has been prohibited for years (in favor of the g_ascii_ version).

Change-Id: I64b7c29099b1c5240757e2026fe3490096a84755
Reviewed-on: https://code.wireshark.org/review/4980
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>