Remove temporary shims (#514)

Motivation:

We added a couple of shims which were upstreamed to swift-nio and were
released in 2.82.0.

Modifications:

- Bump minimum verison of swift-nio
- Remove shims

Result:

Less code

Author: glbrntt

Package.swift

@@ -39,7 +39,7 @@ let package = Package(
         .library(name: "NIOHTTP2", targets: ["NIOHTTP2"])
     ],
     dependencies: [
-        .package(url: "https://github.com/apple/swift-nio.git", from: "2.81.0"),
+        .package(url: "https://github.com/apple/swift-nio.git", from: "2.82.0"),
         .package(url: "https://github.com/apple/swift-atomics.git", from: "1.0.2"),
     ],
     targets: [