Rust: remove shipped feature flag #4584

Job	Run time
Diagnostic export (macos-latest, linked)	1m 27s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 22s
Diagnostic export (macos-latest, nightly-latest)	1m 21s
Diagnostic export (windows-latest, nightly-latest)	2m 56s
Diagnostic export (windows-latest, linked)	3m 26s
Diagnostic export (ubuntu-latest, linked)	1m 16s
	11m 48s

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