Remove IHasTransactionNameSource in v4.0.0

[jamescrosswell]

It appears this interface was slated for removal when it was created... v4.0.0 is our first opportunity to do that.

sentry-dotnet/src/Sentry/IHasTransactionNameSource.cs

Lines 7 to 8 in cd16696

	/// Ideally, this would just be implemented as part of <see cref="ITransaction"/> and <see cref="ITransactionContext"/>.
	/// However, adding a property to a public interface is a breaking change. We can do that in a future major version.