Remove DocC plugin from package manifest

Author: simonjbeaumont

Package.swift

@@ -34,7 +34,6 @@ let package = Package(
     ],
     dependencies: [
         .package(url: "https://github.com/apple/swift-http-types", from: "1.0.0"),
-        .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
     ],
     targets: [
         .target(