Revert "SkFlattenable::Register now performs a sorted insert"

This reverts commit 349f974b1437d2ccef5d773846bc677caa900a70.

Reason for revert: caused tree to break

Original change's description:
> SkFlattenable::Register now performs a sorted insert
>
> In order to maintain the sorted quality of `Entry gEntries`, I updated SkFlattenable::Register to perform a sorted insert with the new element. To avoid possible threading issues, I added an SkSharedMutex.
>
> Change-Id: I14a9001ef2822211bbcbe994b73b4671ae17844d
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1252956
> Reviewed-by: Kaylee Lubick <kjlubick@google.com>
> Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>

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Change-Id: I9bd650ea470ee7f050383e050e5187fb8a845834
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1253856
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>