Locals window can be crowded with return value items when stepping over complex statements

## Issue Description ##
When stepping over complex statements, the Variables pane shows an item for every function stepped over. This can get crowded:


## Steps to Reproduce ##
* Make a statement with lots of different method calls
* Step over

## Expected Behavior ##
I don't want to see the system stuff.

## Actual Behavior ##
Showing the system stuff.

## Environment information ##

**VSCode version**: 1.60.0-insider
**C# Extension**: 1.23.14

Dotnet Information
.NET SDK (reflecting any global.json):
Version: 6.0.100-preview.7.21379.14
Commit: 22d70b47bc

** INCORRECT **
global.json exists:
```
{
"sdk": {
"version": "5.0.400"
}
}
```

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19043
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\6.0.100-preview.7.21379.14\

Host (useful for support):
Version: 6.0.0-preview.7.21377.19
Commit: 91ba01788d

.NET SDKs installed:
5.0.400 [C:\Program Files\dotnet\sdk]
6.0.100-preview.7.21379.14 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.All 2.1.29 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.29 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 6.0.0-preview.7.21378.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.29 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.0-preview.7.21377.19 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.0-preview.7.21378.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs:
https://aka.ms/dotnet-download

Visual Studio Code Extensions

|Extension|Author|Version|
|---|---|---|
|csharp|ms-dotnettools|1.23.14|
|git-graph|mhutchie|1.30.0|
|hexeditor|ms-vscode|1.8.2|
|hl7tools|RobHolme|1.13.2|
|material-icon-theme|PKief|4.10.0|
|msbuild-project-tools|tintoy|0.3.15|
|night-owl|sdras|2.0.1|
|quicktype|quicktype|12.0.46|
|TabOut|albert|0.2.1|
|vscode-base64|adamhartford|0.1.0|
|vscode-thunder-client|rangav|1.9.1|
|vscodeintellicode|VisualStudioExptTeam|1.2.14|;