generator: Skip setters on `returnedonly="true"` structs #3086

Job	Run time
Check	42s
Clippy (x86_64-pc-windows-msvc)	2m 6s
Check ash, ash-window and ash-rewrite MSRV (1.69.0)	24s
Build-test docs	42s
Clippy (x86_64-unknown-linux-gnu)	2m 15s
Rustfmt	4s
Clippy (x86_64-apple-darwin)	2m 9s
Clippy (aarch64-apple-ios)	23s
Generated	34s
Test Suite	2m 10s
	11m 29s

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