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Also remove extensions_web_apps_ provider.

This CL introduces no behavior changes.

In next CL: Clean up ExtensionApps class as if it manages kExtension
apps only.

Bug: 1065748
Change-Id: Ibb2a41db9d0ed23ff4d5d572298dc29a02515077
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2520476Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824644}