Use built-in SDK features for commit hash attributes and source-link

[natemcmaster]

MSBuild and Microsoft.NET.Sdk will soon have the ability to embed commit hashes and other source info. Once that is available, we can probably drop our own implementations of commit hash attribute generation, source link generation and others.

Currently a WIP - cref NuGet/NuGet.Client#2081, dotnet/msbuild#3063, https://github.com/tmat/repository-info/blob/master/docs/Readme.md by @tmat.

[natemcmaster]

https://github.com/dotnet/designs/blob/master/accepted/build/standard-ci-env-variables.md

[dougbu]

You mentioned dotnet/aspnetcore#7504 was part of this issue. What remains?

[natemcmaster]

Delete obsolete code. :)