Remove kEnableSecondaryAccountsForChild flag

This feature launched in M86 and it is time for cleanup.

Bug: 1140500
Change-Id: Ica611b417ef7da88e86606cbeb8f0b83903d0c49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2651434


Commit-Queue: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Cait Phillips <caitkp@chromium.org>
Reviewed-by: Anastasiia N <anastasiian@chromium.org>
Reviewed-by: Yilkal Abe <yilkal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#850436}