Rust: remove shipped feature flag #4402

Job	Run time
Diagnostic export (windows-latest, linked)	2m 49s
Diagnostic export (macos-latest, linked)	1m 34s
Diagnostic export (macos-latest, nightly-latest)	1m 22s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 6s
Diagnostic export (windows-latest, nightly-latest)	4m 46s
Diagnostic export (ubuntu-latest, linked)	1m 2s
	12m 39s

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