Remove monomod dev build shims

This removes shims in-place to fetch monomod from outside of nuget,
since monomod has pushed the applicable versions to their primary nuget
repositories, and because the dev builds add unnecessary complexity to
the CI pipeline if kept in place.

appveyor.yml

@@ -5,8 +5,6 @@ build_script:
 - ps: >-
     git submodule update --init --recursive
 
-    dotnet nuget add source https://pkgs.dev.azure.com/MonoMod/MonoMod/_packaging/DevBuilds%40Local/nuget/v3/index.json -n DevBuilds@Local
-
     dotnet build TShock.sln
 
     dotnet test