basestore: use tryunlink from mercurial.util to try and remove files
This commit just refactors to use a method meant for the same
purpose. Thanks to @quark for discovering this method!
Test Plan: Ran all the tests.
Reviewers: #fbhgext, quark
Reviewed By: #fbhgext, quark
Differential Revision: https://phab.mercurial-scm.org/D1571