Drop unavailable `private_in_public` linter

[ansasaki]

When compiling the agent with nightly compiler for test coverage measurement, we get the following warning:

Compiling keylime_agent v0.2.2 (/var/ARTIFACTS/work-e2engliyx41/packit-ci/e2e/tree/keylime-agent)

warning: lint `private_in_public` has been removed: replaced with another group of lints, see RFC <https://rust-lang.github.io/rfcs/2145-type-privacy.html> for more information

  --> keylime-agent/src/main.rs:13:5

   |

13 |     private_in_public,

   |     ^^^^^^^^^^^^^^^^^

   |

   = note: `#[warn(renamed_and_removed_lints)]` on by default