zfs: Remove unused abd_alloc_scatter_offset_chunkcnt

[rlibby]

Summary

From openzfs-master bf156c966 commit message:

Remove function that become unused after refactoring in
e2af2acce3436acdb2b35fdc7c9de1a30ea85514.

Cherry-picked from openzfs bf156c966bd68aef3b08716f9e81238a4571236a
Patch Author: Ryan Libby <[e-mail]>

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I posted this review to make sure this is okay in terms of process and commit log. This is a diff & patch, and the above is the intended commit message. I used recent commits by @mm as a template, but I see a few different styles in recent history in git log sys/contrib. This is the upstream commit:
https://github.com/openzfs/zfs/commit/bf156c966bd68aef3b08716f9e81238a4571236a

[mm]

I have updated the vendor branch to bf156c966, so technically we can merge from it. The idea is that we keep as low diff to upstream as possible. Have you already done merging from a vendor branch?

[mm]

I have merged lastest OpenZFS to main including the new "compatibility" property, you can close this now. Thanks for getting the change upstream!

[rlibby]

In D28768#644214, @mm wrote:

I have merged lastest OpenZFS to main including the new "compatibility" property, you can close this now. Thanks for getting the change upstream!

Great, thanks!

In D28768#644120, @mm wrote:

I have updated the vendor branch to bf156c966, so technically we can merge from it. The idea is that we keep as low diff to upstream as possible. Have you already done merging from a vendor branch?

I have not. I'll keep this in mind for future changes.