[ui] WorkspaceView: Remove Load Model button from Viewer3D

[cbentejac]

Description

This PR removes the "Load Model" button that exists in the code for the 3D Viewer. It isn't used anymore, and has moreover faulty visibility conditions. It should thus be removed entirely.

Fixes #1126.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.46%. Comparing base (ad20cc5) to head (a850550).
⚠️ Report is 7 commits behind head on develop.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2842   +/-   ##
========================================
  Coverage    77.46%   77.46%           
========================================
  Files           48       48           
  Lines         6735     6735           
========================================
  Hits          5217     5217           
  Misses        1518     1518           

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