Remove enablePowerFxOverlay from Test Settings

[landonmsft]

Description

enablePowerFxOverlay is a setting that we're not implementing in Test Engine. It appears that it was intended to be implemented at some point, but never was. Removing it for now.

Checklist

• [x] The code change is covered by unit tests. I have added tests that prove my fix is effective or that my feature works
• [x] I have performed end-to-end test locally.
• [x] New and existing unit tests pass locally with my changes
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I used clear names for everything
• [x] I have performed a self-review of my own code

[github-actions[bot]]

Package	Line Rate	Branch Rate	Complexity	Health
Microsoft.PowerApps.TestEngine	92%	88%	922	✔
Summary	92% (2455 / 2683)	88% (557 / 634)	922	✔

Minimum allowed line rate is 85%