remove NIO1 API shims (#1897)

Motivation:

The lastest NIO versions require Swift 5.2+ to compile. Given that
hopefully nobody ever created a NIO1 application for Swift 5.2+, I'd say
it's about time to remove the NIO1 API shims (and related docs).

Modification:

- remove the NIO1 API shims
- remove the migration docs

Result:

Less code.

Package.swift

@@ -16,8 +16,6 @@
 import PackageDescription
 
 var targets: [PackageDescription.Target] = [
- .target(name: "_NIO1APIShims",
- dependencies: ["NIO", "NIOHTTP1", "NIOTLS", "NIOFoundationCompat", "NIOWebSocket"]),
  .target(name: "NIO",
  dependencies: ["CNIOLinux",
  "CNIODarwin",
@@ -89,7 +87,6 @@ let package = Package(
  name: "swift-nio",
  products: [
  .library(name: "NIO", targets: ["NIO"]),
- .library(name: "_NIO1APIShims", targets: ["_NIO1APIShims"]),
  .library(name: "_NIOConcurrency", targets: ["_NIOConcurrency"]),
  .library(name: "NIOTLS", targets: ["NIOTLS"]),
  .library(name: "NIOHTTP1", targets: ["NIOHTTP1"]),