tools/valgrind/asan: drop unnecessary py2 compatibility

This is blocked by https://crbug.com/1293351

Bug: 1275016
Change-Id: I57aea6e9d210fa42b646c7993917d71b246929e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3428262
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#971975}
NOKEYCHECK=True
GitOrigin-RevId: d63f7e92c4a34cb6dadeb79aea8c7f6d39c1071f