Eliminate packages project

[BruceForstall]

The jitutils build.cmd has a -f option that does a restore on dummy project to create a directory with a set of libraries assemblies that can be used for asm diffs.

This shouldn't be necessary anymore, as you can just use a dotnet/runtime Core_Root directory for the same purpose.

Proposal: remove the -f build flag, and the src\packages directory.

@dotnet/jit-contrib Feedback? Is there a use for this that justifies keeping it?