viewer.py - disable Simulator default agent navmesh override

[aclegg3]

Motivation and Context

#2111 introduced default agent navmesh setup through settings.py. The viewer application already has logic for this and using the default agent settings will override SceneDataset configured navmeshes if the config is non-default. It should therefore be disabled in the app.

How Has This Been Tested

Local

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.