Revert "AW: enable SiteBlock feature by default"

This reverts commit 88b39102d586093e73ff0d329ead54334da940bc.

Reason for revert: leadership guidance is to pause the launch until Jan

Original change's description:
> AW: enable SiteBlock feature by default
>
> This flips the flag for WebViewSupervisedUserSiteBlock. Metrics for the
> change are mostly neutral to positive and this change should be ready to
> launch.
>
> This does not clean up the feature flag. Code cleanup will happen later
> after a full launch (as per best practices) and the old code path will
> continue to be supported until that point in time.
>
> Bug: 362574154
> Test: N/A
> Change-Id: I352b6d4feef82ed4f8376975f82db432d011f377
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6099112
> Commit-Queue: Nate Fischer <ntfschr@chromium.org>
> Auto-Submit: Nate Fischer <ntfschr@chromium.org>
> Reviewed-by: Peter Pakkenberg <pbirk@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1397381}

Bug: 362574154
Change-Id: I3ff25f4f5252c3ca0910b43bc28d87d9f2608c94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6106521
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Adam Walls <avvall@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1398200}