Remove montgomery reduction

[GiacomoPope]

Pretty decent speed up for just deleting code:

Jack: kyber-py % python3 benchmark_kyber.py                 
----------------------------------------------------------------------
 Params    | keygen | keygen/s | encap  | encap/s  | decap  | decap/s
----------------------------------------------------------------------
 Kyber512  |  5.59ms    178.75    8.32ms    120.12   13.40ms     74.63
 Kyber768  |  8.10ms    123.53   11.82ms     84.58   18.78ms     53.26
 Kyber1024 | 11.71ms     85.39   16.30ms     61.37   25.45ms     39.29
Jack: kyber-py % git checkout remove_montgomery_reduction
Switched to branch 'remove_montgomery_reduction'
Jack: kyber-py % python3 benchmark_kyber.py              
----------------------------------------------------------------------
 Params    | keygen | keygen/s | encap  | encap/s  | decap  | decap/s
----------------------------------------------------------------------
 Kyber512  |  4.85ms    206.27    7.07ms    141.46   11.59ms     86.31
 Kyber768  |  6.93ms    144.21   10.11ms     98.94   16.49ms     60.63
 Kyber1024 |  9.88ms    101.19   13.76ms     72.67   22.22ms     45.01