[Tooling] Remove unused function setRestoreWorkingDir

The last use was removed by:

  commit 146ec74a
  Author: Jan Svoboda <jan_svoboda@apple.com>
  Date:   Fri Sep 10 10:24:16 2021 +0200

Once I remove the function, RestoreCWD is always true, so this patch
removes the variable and propagates the constant.

Differential Revision: https://reviews.llvm.org/D151786