Remove obsolete members

[jamescrosswell]

We have various [Obsolete] attributes scattered through the code mentioning things that will be removed in v4.0.0.

Here's one example, best to run a search though as there are more:

sentry-dotnet/src/Sentry/SentryOptions.cs

Line 708 in ad87961

[Obsolete("Use ReportAssembliesMode instead", error: false)]