Remove reference to external solutions

Author: StanleyGoldman

docs/contributing/how-to-build.md

@@ -68,9 +68,3 @@ The `GitHub.Unity.sln` solution includes several projects:
 - GitHub.Logging: A logging helper library
 - GitHub.Api: The core of the extension. This project is C#6 and includes async/await threading and other features that Unity cannot currently compile.
 - GitHub.Unity: Unity-specific code. This project is compilable by Unity
-
-## Other solutions
-
-Octokit is normally included as a prebuilt DLL (in the `lib` directory), but when updating Octokit or debugging issues with the 3.5 backport, it can sometimes be handy to reference the project directly. The `GitHub.Unity.OctokitDebugging.sln` solution is a helper solution that includes Octokit and dotnet-httpclient35 as referenced projects instead of prebuilt dlls.
-
-This solution requires a clone of [https://github.com/github-for-unity/octokit.net](https://github.com/github-for-unity/octokit.net) and [https://github.com/github-for-unity/dotnet-httpclient35](https://github.com/github-for-unity/dotnet-httpclient35) to exist next to your GitHub for Unity checkout.