Remove obsolete private_in_public lint in nightly.

[thenorili]

RFC 2145 is on nightly now, deprecating the public_in_private lint.

https://rust-lang.github.io/rfcs/2145-type-privacy.html

See PR #113

public_in_private has been superceded by three new lints. The first two are warn-by-default and the third is allow-by-default. See the excerpt below for some details.

Lint private_interfaces is reported when a type with visibility x is used in primary interface of an item with effective visibility y and x < y. This lint is warn-by-default.
Lint private_bounds is reported when a type or trait with visibility
x is used in secondary interface of an item with effective
visibility y and x < y. This lint is warn-by-default.

Lint unnameable_types is reported when effective visibility of a
type is larger than module in which it can be named, either
directly, or through reexports, or through trivial type aliases
(type X = Y;, no generics on both sides). This lint is
allow-by-default.

Compatibility lint private_in_public is never reported and removed.

[thenorili]

PR #113 is merged, this is closed.