remove useless code in lib/ui/compositing/scene_builder.cc

[xxrlzzz]

This two lines of code was add in #6256 and now is useless since line 205-207 was deleted in some commit.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

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[xxrlzzz]

@googlebot I signed it!

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