Don't require connection string during migration remove

[atrauzzi]

I'm looking to remove a migration I created in a class library. I was able to create it without requiring a viable DbContext, yet when I go to remove the last migration I just created, the EF Core tools crash because they aren't being triggered from a project that configures the context.

Ideally, I'd like to just be able to use the CLI tools without having any notion of a running database be presumed, as that's just not a necessity.