generator: Skip setters on `returnedonly="true"` structs #3087

Job	Run time
Check ash, ash-window and ash-rewrite MSRV (1.69.0)	22s
Test Suite	1m 17s
Check	42s
Build-test docs	48s
Generated	33s
Rustfmt	4s
Clippy (x86_64-pc-windows-msvc)	1m 40s
Clippy (x86_64-unknown-linux-gnu)	1m 14s
Clippy (x86_64-apple-darwin)	2m 7s
Clippy (aarch64-apple-ios)	21s
	9m 8s

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