Rust: remove shipped feature flag #4411

Job	Run time
Diagnostic export (macos-latest, linked)	1m 2s
Diagnostic export (ubuntu-latest, linked)	1m 2s
Diagnostic export (windows-latest, linked)	3m 47s
Diagnostic export (macos-latest, nightly-latest)	1m 5s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 1s
Diagnostic export (windows-latest, nightly-latest)	2m 51s
	10m 48s

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