treewide: cleanup cargo-deny warnings

Clean up a few dead `cargo-deny` warnings.

Change-Id: Ib60d1726d943e6dba365f8bc3ffafdfaa52b6474
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/6971
Tested-by: Buildkite CI
Reviewed-by: Ethan Donowitz <ethan@readyset.io>

deny.toml

@@ -48,8 +48,6 @@ notice = "warn"
 # A list of advisory IDs to ignore. Note that ignored advisories will still
 # output a note when they are encountered.
 ignore = [
-    # Mitigated by disallowed-methods in clippy.toml
-    "RUSTSEC-2020-0071",
 ]
 # Threshold for security vulnerabilities, any vulnerability with a CVSS score
 # lower than the range specified will be ignored. Note that ignored advisories