Add warning for unused license exceptions

[repi]

Had a repo with a license exception like this:

exceptions = [
    { allow = ["Zlib"], name = "adler32" },
    { allow = ["CC0-1.0"], name = "constant_time_eq" },
    { allow = ["CC0-1.0"], name = "notify" },
    { allow = ["CC0-1.0"], name = "x11-dl" },
]

But then removed the dependency on the notify crate, so the license exception wasn't used anymore. Would be good if cargo-deny warns about unused licensed exceptions similar to how it warns about unused ban skips, but likely even more important for licenses as one wants to keep the license exception list as minimal and clean as possible.