Revert "Add fieldtrial config for AllowHistoryDeletionForChildAccounts"

This reverts commit df5e491f81b998745ffe013b53649817411b02a7.

Reason for revert: HistoryActivityTest#testSupervisedUser is flaky

Original change's description:
> Add fieldtrial config for AllowHistoryDeletionForChildAccounts
>
> This adds the fieldtrial config, in preparation for starting to enable
> this on beta.
>
> I've updated the one test which fails as a result, splitting it into two
> for now so that we have coverage of both cases (I've added a TODO to
> remove the disabled case as part of the cleanup bug).
>
> Bug: 1318343
> Change-Id: I19735a09bf1621e5202071734e5e41ac16e5cef6
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3599070
> Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
> Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
> Commit-Queue: James Lee <ljjlee@google.com>
> Auto-Submit: James Lee <ljjlee@google.com>
> Cr-Commit-Position: refs/heads/main@{#995089}

Change-Id: I15b348a5aeb33132df2388db05c0cd7778dcca84
Fixed: 1319001
Bug: 1318343
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3603233
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Owners-Override: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#995392}