Remove outdated v3 code #1752

Job	Run time
cache-pixi-lock / Generate output	25s
TypeChecking: pixi run typing	1s
merge-test-artifacts	1s
Integration: ${{ matrix.os }} \| pixi run -e ${{ matrix.pixi-environment }} tests-notebooks	1s
Unit tests: ${{ matrix.os }} \| pixi run -e ${{ matrix.pixi-environment }} tests	1s
	29s

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