Remove a bogus make-dissector-reg dependency from UseMakeDissectorReg.cmake

The shell script is not used in the CMake build system.

Change-Id: I97d3d750f2521c626594200da8fc9a4453414576
Reviewed-on: https://code.wireshark.org/review/3530
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>