Syn is no longer duplicated

Remove exception.  This issue seems to have been resolved for a while.

Signed-off-by: Daniel Noland <daniel@githedgehog.com>

deny.toml

@@ -190,10 +190,6 @@ skip-tree = [
     #{ crate = "ansi_term@0.11.0", depth = 20 },
 ]
 
-[[bans.skip]]
-name = "syn"
-reason = "syn is basically always a build time dependency so we aren't shipping multiple versions"
-
 # This section is considered when running `cargo deny check sources`.
 # More documentation about the 'sources' section can be found here:
 # https://embarkstudios.github.io/cargo-deny/checks/sources/cfg.html