Remove ConfigurationBuilder.Add(provider, load) overload

[divega]

A bunch of tests call this overload passing an in-memory provider and load: false but this seems complete superfluous as MemoryConfigurationProvider.Load() is a no-op anyway.

It doesn't seem to be used anywhere and IConfigurationBuilder doesn't define it. So it is most likely just leftover from a previous design iteration that we can get rid of.

[divega]

Unless @lodejard remember a reason we need it.

[HaoK]

30e59f1