[Responsiveness] Remove implicit RPC from background thread via unneeded GetServiceAsync extensions

[nkolev92]

These IAsyncServiceProvider extension methods/overloads are unnecessarily, can increase thread-pool latency and should be deleted in lieu of the generic built-in extension methods from the SDK. Care should be taken to handle any behavior differences, such as these built-in extension methods throwing by default when the service does not exist.

Background:

These extension methods perform a cast on the result of IAsyncServiceProvider.GetServiceAsync, which if occurred on a background thread, can perform a blocking COM RPC to the UI thread for STA-bound objects, such as IVsSolution, DTE, etc. This will block a thread pool thread until the UI thread responds, decreasing the number of available threads, which in turn can lead to UI responsiveness issues[1]. The built-in overloads perform a non-blocking wait to the UI thread to perform the cast, and therefore do not suffer from this problem.

1336518