Delete the obsolete "App Registry" design

That design was introduced, some months ago (2018 July), in
crrev.com/c/1127513 "Create a skeleton App Service and App Registry".
The 'skeleton' App Registry wasn't actually called anywhere in
production code, only in tests.

Some months later (2018 November), crrev.com/c/1328626 "Add
chrome/services/app_service design doc" and crrev.com/c/1338590 "Add an
App Service registry implementation" gave more detail or 'flesh on the
bones' about how the App Service (and the registry aspect of that
service) would work. This involved a new design, also called the "App
Registry". The old design was deprecated.

This commit deletes that old code, removing the confusion in having both
the old and new designs, both called "App Registry".

The new design does not depend on the prefs service, so some
dependencies were removed. In the future, some other to-be-written part
of the App Service may depend on the prefs service. At that future point
in time, some of these dependencies (and the 'pause incoming Mojo
requests until the pref service is ready' dance in app_service.cc) may
be re-introduced. But for now, YAGNI.

BUG=826982

Change-Id: I4ad52c5b9ba307a3922ed6dda330b7d9aedf29a0
Reviewed-on: https://chromium-review.googlesource.com/c/1348881
Commit-Queue: Nigel Tao <nigeltao@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611579}
11 files changed