Don't switch to UI thread to dispose workspace project context

[drewnoakes]

In dotnet/roslyn#51016 @jasonmalinowski explains that disposal of IWorkspaceProjectContext is free threaded.

Microsoft Reviewers: Open in CodeFlow

[davkean]

@jasonmalinowski Just to be clear, and the reason I added this switch back, this stack no longer exists?

Microsoft.CodeAnalysis.Workspaces.dll!Roslyn.Utilities.Contract.ThrowIfFalse(bool condition, string message)    Unknown
    Microsoft.CodeAnalysis.EditorFeatures.dll!Microsoft.CodeAnalysis.Editor.Shared.Utilities.ForegroundThreadAffinitizedObject.AssertIsForeground()    Unknown
    Microsoft.VisualStudio.LanguageServices.dll!Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem.VisualStudioWorkspaceImpl.SetDocumentContext(Microsoft.CodeAnalysis.DocumentId documentId)    Unknown
    Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.Workspace.ClearOpenDocument(Microsoft.CodeAnalysis.DocumentId documentId, bool isSolutionClosing)    Unknown
    Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.Workspace.ClearOpenDocuments(Microsoft.CodeAnalysis.ProjectId projectId)    Unknown
    Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.Workspace.ClearProjectData(Microsoft.CodeAnalysis.ProjectId projectId)    Unknown
    Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.Workspace.OnProjectRemoved(Microsoft.CodeAnalysis.ProjectId projectId)    Unknown
    Microsoft.VisualStudio.LanguageServices.dll!Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem.VisualStudioWorkspaceImpl.OnProjectRemoved(Microsoft.CodeAnalysis.ProjectId projectId)    Unknown
    Microsoft.VisualStudio.LanguageServices.dll!Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem.VisualStudioProject.RemoveFromWorkspace.AnonymousMethod__86_0(Microsoft.CodeAnalysis.Workspace w)    Unknown
    Microsoft.VisualStudio.LanguageServices.dll!Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem.VisualStudioWorkspaceImpl.ApplyChangeToWorkspace(System.Action<Microsoft.CodeAnalysis.Workspace> action)    Unknown
    Microsoft.VisualStudio.LanguageServices.dll!Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem.VisualStudioProject.RemoveFromWorkspace()    Unknown
    Microsoft.VisualStudio.LanguageServices.Implementation.dll!Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem.CPS.CPSProject.Dispose()    Unknown

[jasonmalinowski]

@davkean: dotnet/roslyn#31019 (comment) -- we fixed this awhile ago.