Remove obsolete private_in_public lint in nightly. #291

Job	Run time
Miri	1m 55s
Rustfmt (stable)	5s
Clippy (stable)	19s
Check (stable)	25s
Test Suite	6m 16s
Test Suite (stable)	32s
Test Suite (ubuntu-latest)	55s
Test Suite (--no-default-features)	6m 0s
Test Suite (stable, --no-default-features)	1m 7s
Test Suite (windows-latest)	1m 4s
Test Suite (--features track-caller)	4m 2s
Test Suite (stable, --features track-caller)	47s
Test Suite (macOS-latest)	1m 42s
Test Suite (--features auto-install)	3m 56s
Test Suite (stable, --features pyo3)	1m 4s
Test Suite (stable, --features auto-install)	32s
Test Suite (stable, --all-features)	1m 11s
Test Suite (beta)	1m 3s
Test Suite (beta, --no-default-features)	1m 2s
Test Suite (beta, --features track-caller)	32s
Test Suite (beta, --features pyo3)	36s
Test Suite (beta, --features auto-install)	1m 3s
Test Suite (beta, --all-features)	1m 10s
Test Suite (nightly)	44s
Test Suite (nightly, --no-default-features)	1m 31s
Test Suite (nightly, --features track-caller)	1m 32s
Test Suite (nightly, --features pyo3)	44s
Test Suite (nightly, --features auto-install)	43s
Test Suite (nightly, --all-features)	1m 16s
	43m 48s

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