[keyserver] Remove test-threadstore-reductions script

Summary:
Context: https://linear.app/comm/issue/ENG-5711/remove-test-threadstore-reductionsjs-file

Depends on D9841

Test Plan: NA

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D9842