VS Code extension: Do not warn about unused `using`s

The C# extension and VS Code already show recommendations to remove unused usings in any open C# file. However the warnings for this same cleanup recommendation make it more difficult to find real issues in the project. Note projects w/ unused usings build without warnings.

The warnings in the IDE are inconsistent as well: VS code doesn’t even show a recommendation to simplify the namespace qualification in the statement below.

using System;
using System.Threading.Tasks;

...
    var result = System.Threading.Tasks.Task.FromResult(0);

For example, in a recently-created project using a VS template:

[DustinCampbell]

It is annoying. Agreed 100%. 😄

I (finally) filed an official request on VS Code for faded code: https://github.com/Microsoft/vscode/issues/20219.

[DustinCampbell]

(Very glad to see you here @Eilon 😄)