remove setNumProbes (facebookresearch#2797)

Summary:
Pull Request resolved: facebookresearch#2797

This is the last code instance of setNumProbes
Removing because some people still seem run into errors due to this.

Reviewed By: algoriddle

Differential Revision: D44421600

fbshipit-source-id: fbc1a9d49a0175ddf24c32dab5c1bdb5f1bbbac6

Author: mdouze

benchs/bench_gpu_sift1m.py

@@ -85,7 +85,8 @@
 
 for lnprobe in range(10):
     nprobe = 1 << lnprobe
-    index.setNumProbes(nprobe)
+    index.nprobe
+    index.nprobe = nprobe
     t, r = evaluate(index, xq, gt, 100)
 
     print("nprobe=%4d %.3f ms recalls= %.4f %.4f %.4f" % (nprobe, t, r[1], r[10], r[100]))