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This CL removes unused code, and does not change the
TestLauncher behavior.

This code was introduced here https://codereview.chromium.org/1806353003
to allow launching unit tests in Mash.

The Mash tests launcher no longer exist, and no other implementation of
PreRunTest exist.

Bug: 936248
Change-Id: Ie7e968d52904b249d57f23dc7c404d477265ed47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626023
Commit-Queue: Ilia Samsonov <isamsonov@google.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663778}