Remove the light GPU implementation (aka FaustGPU) as it was an experimental implementation based on gpu_mod (while GPU2 is the full implementation).

Before removing I verified there was no loss in performance (with pyfaust-3.0.17).

(latest_pyfaust) hhadjdji@v100alpha:~$ python ./test_gpu_mod.py | grep -v changed
F.shape (1024, 1024) F.numfactors: 5 fac_type= sparse
***toarray*** cpu time: 1.6364107518456876 FaustGPU time: 1.6207437822595239 GPU2 time: 0.609403109177947
***F@M*** cpu time: 2.066476149018854 FaustGPU time: 1.0155699900351465 GPU2 time: 0.8688869220204651
***F.norm(2)*** cpu time: 0.03744047088548541 FaustGPU time: 0.20482274424284697 GPU2 time: 0.19360520970076323
F.shape (1024, 1024) F.numfactors: 5 fac_type= dense
***toarray*** cpu time: 6.241591406054795 FaustGPU time: 6.68755641579628 GPU2 time: 0.23564746510237455
***F@M*** cpu time: 7.5440095462836325 FaustGPU time: 1.0803766320459545 GPU2 time: 0.9672970180399716
***F.norm(2)*** cpu time: 4.2132647559046745 FaustGPU time: 0.24246237566694617 GPU2 time: 0.24203181779012084

The script is hosted on gitlab : https://gitlab.inria.fr/faustgrp/faust/-/snippets/708