Fix crash on close

[Al12rs]

This was due to WorkspaceViewModel unregistering itself from WorkspaceController.AllWorkspaces when it is deactivated (WhenActivated DisposeWith).

This changed the number of children during the OnDetachedFromVisualTree calls.
Removing the call to UnregisterWorkspace in the disposable here fixes the crash on close.

Currently we don't actually have any way to remove workspaces, as there is no way to delete a loadout. So for now the removal code is inactive. We will figure out how that works when we support deleting loadouts.

Part of #906.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.75%. Comparing base (5c4ef18) to head (869270b).
Report is 82 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1068      +/-   ##
==========================================
- Coverage   54.82%   54.75%   -0.07%     
==========================================
  Files         662      662              
  Lines       21997    22008      +11     
  Branches     1717     1704      -13     
==========================================
- Hits        12059    12050       -9     
- Misses       9541     9561      +20     
  Partials      397      397              

Flag	Coverage Δ
Linux	54.12% <100.00%> (-0.03%)	⬇️
Windows	54.03% <100.00%> (-0.03%)	⬇️
clean_environment_tests	54.74% <100.00%> (-0.07%)	⬇️
macOS	?

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Files	Coverage Δ
...UI/WorkspaceSystem/Workspace/WorkspaceViewModel.cs	47.75% <100.00%> (-0.69%)	⬇️
...eSystem/WorkspaceController/WorkspaceController.cs	35.80% <100.00%> (-0.28%)	⬇️

... and 12 files with indirect coverage changes