remove std_single and var_single code in TH (#31608)
Summary:
std_single and var_single in TH never be used, remove them.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31608
Differential Revision: D19270920
Pulled By: VitalyFedyunin
fbshipit-source-id: e106a42383bf224f7e2c1c092b95484d23af4b0a
