Remove TfNative Smart-Dim model and code-path

Remove kUserActivityPredictionMlService that is launched to 'true'.

* Remove feature flag
* Remove UI for feature flag (about_flags)
* Remove code paths followed in the 'false' case.
* Remove TfNative generated code and preprocessor config.
* Update unit tests.

Bug: 893425, 942934
Change-Id: I14e40e3cee0e8285f9f2bd018a9830e7071d8084
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1605585
Commit-Queue: Andrew Moylan <amoylan@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Jia Meng <jiameng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661273}
13 files changed