Remove static task runner from ActivateAffiliationBasedMatching.

The original problem isn't a problem anymore. The affiliation service is
created once for all the profiles on start-up.

The static task runner is bad because it's never destroyed. That may be
the reason for test flakiness. The following error appears in unrelated
tests:
TaskEnvironment: RunTask took more than 30 seconds. Posted from
Initialize@components/password_manager/core/browser/android_affiliation/
android_affiliation_service.cc:45

Bug: 786157
Change-Id: Idac9b7b23d3fdda756eb5f95051e53b703a994b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3122094


Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915905}