Now that Python is mandatory on Windows, remove checks for it from nmakefiles

Change-Id: I2ca6abb372ec4bda0af1aa40089082533a61df3a
Reviewed-on: https://code.wireshark.org/review/3392
Petri-Dish: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>