Revert "[ProfilePicker] Enable picker on startup only for active users"

This reverts commit 82afac04671a8e17f6cf6b46956c8c8e07adf2d3.

Reason for revert: This CL broke StartupBrowserCreatorPickerTest.TestSetup.
 
Bug: 1163675

Original change's description:
> [ProfilePicker] Enable picker on startup only for active users
>
> Keep the picker disabled on startup until:
> - multiple profiles were used in the last 28 days, or
> - the picker has been shown once
>
> A new prefs::kBrowserProfilePickerShown pref is added to track whether
> the picker has been shown before.
>
> Bug: 1063856
> Change-Id: Ia35a3b2212b3811ed592d81fa516bc1ffc07accc
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2598848
> Commit-Queue: Alex Ilin <alexilin@chromium.org>
> Reviewed-by: David Roger <droger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#840598}

TBR=droger@chromium.org,alexilin@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: I306f0b7fa225b9d4043e948ec4c7a15e82ed9859
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1063856
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612309
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840869}