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Remove exposed plugin API since all logic is now contained to a single
file and it runs immediately upon injection.

The unittests are now obsolete because no API is exposed. However, all
previous unittest scenarios are covered by the PluginPlaceholderTestCase
EarlGrey tests.

Bug: 802344
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I2f31af1fe9fa4f94f678064fe2a8cc316b837757
Reviewed-on: https://chromium-review.googlesource.com/899914
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534169}