We no longer require the docc plugin

Motivation:

Swift Package Index builds our docs for us and will automatically insert
the dependency.

Modifications:

- Remove the docc-plugin dependency

Result:

Fewer dependencies

Package.swift

@@ -23,7 +23,6 @@ let package = Package(
     dependencies: [
         .package(url: "https://github.com/apple/swift-nio.git", from: "2.62.0"),
         .package(url: "https://github.com/apple/swift-atomics.git", from: "1.0.2"),
-        .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
     ],
     targets: [
         .target(