Remove outdated RUSTSEC exemption for macOS

This got fixed with #136 but the exemption for the mach crate survived
but is no longer needed. Keeping it in place would prevent us from
getting notified from CI in case this dependency would be reintroduced
by accident.

deny.toml

@@ -50,7 +50,6 @@ notice = "deny"
 # A list of advisory IDs to ignore. Note that ignored advisories will still
 # output a note when they are encountered.
 ignore = [
-    "RUSTSEC-2020-0168", # caused by unmaintained mach dependency of core-foundation-sys
     "RUSTSEC-2021-0145", # caused by unmaintained atty
 ]
 # Threshold for security vulnerabilities, any vulnerability with a CVSS score