Revert threadpool minWorkSize change on iOS
Revert threadpool minWorkSize change on iOS

Reviewed By: sf-wind

Differential Revision: D5380298

fbshipit-source-id: fdf98bdda30e8cd6689c59fcc0357bca129d409b

davinwang/caffe2,xzturn/caffe2,bwasti/caffe2,sf-wind/caffe2,davinwang/caffe2,sf-wind/caffe2,pietern/caffe2,davinwang/caffe2,pietern/caffe2,bwasti/caffe2,bwasti/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,sf-wind/caffe2,bwasti/caffe2,sf-wind/caffe2,pietern/caffe2,Yangqing/caffe2,sf-wind/caffe2,xzturn/caffe2,pietern/caffe2,davinwang/caffe2,Yangqing/caffe2,davinwang/caffe2,pietern/caffe2,xzturn/caffe2,bwasti/caffe2,caffe2/caffe2,Yangqing/caffe2,Yangqing/caffe2