Enable clippy::unit_arg (#8742)

This PR enables the
[`clippy::unit_arg`](https://rust-lang.github.io/rust-clippy/master/index.html#/unit_arg)
rule and suppresses the false positive that it flags.

Release Notes:

- N/A

tooling/xtask/src/main.rs

@@ -121,7 +121,6 @@ fn run_clippy(args: ClippyArgs) -> Result<()> {
         "clippy::single_range_in_vec_init",
         "clippy::suspicious_to_owned",
         "clippy::type_complexity",
-        "clippy::unit_arg",
         "clippy::unnecessary_operation",
         "clippy::unnecessary_to_owned",
         "clippy::unnecessary_unwrap",