Remove //device/devices_app.

This change removes the code to create a Mojo app containing device
services (currently only USB) and instead makes the Mojo bindings for
//device/usb part of that directory.

The Mojo service implementation is still in a separate //device/usb/mojo
target but this will eventually merge into its parent directory as the
Mojo interface becomes the only public interface for //device/usb.

BUG=None

Review URL: https://codereview.chromium.org/1682363002

Cr-Commit-Position: refs/heads/master@{#374778}
52 files changed