gh-41004: remove one deprecation in schemes
after
File: src/sage/schemes/projective/projective_morphism.py ; PR: #29145
closed in 2020
Note: this was more like a warning, so there is no function removed now.
### :memo: Checklist
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- [x] The description explains in detail what this PR is about.
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URL: https://github.com/sagemath/sage/pull/41004
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert, John Cremona