Don't show the echo/python command lines when making plugin.c

For consistency with epan/dissectors/Makefile.am. Also, remove the
"with python" parts, since that's the only option now.

Change-Id: I761e1bf7995c1cc1ebd790013181fd6116b289a1
Reviewed-on: https://code.wireshark.org/review/3925
Reviewed-by: Evan Huus <eapache@gmail.com>
Petri-Dish: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>