Don't synthesize cargo manifest for licenses

[Jake-Shadle]

While we no longer synthesize fake manifests for the bans.wildcards check as of #673, we still do that for licenses. This doesn't really make sense, as we will have a license field in a majority of cases, and in the case the user overrides the SPDX expression we would use the expression in the deny config, or if it is automatically inferred by ANDing the detected license files, that should be clearly pointed out in a different way.