Rust: remove shipped feature flag #4694

Job	Run time
Diagnostic export (ubuntu-latest, nightly-latest)	1m 13s
Diagnostic export (macos-latest, nightly-latest)	1m 16s
Diagnostic export (windows-latest, nightly-latest)	3m 1s
Diagnostic export (macos-latest, linked)	1m 10s
Diagnostic export (windows-latest, linked)	3m 13s
Diagnostic export (ubuntu-latest, linked)	1m 8s
	11m 1s

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