Remove unneccessary method DesktopModeStatus#isDesktopModeFlagEnabled.

This method was used to change flag value within a test. We dont need to
have tests that do that,  as flags are process stable in real scenarios.

Test: current tests pass
Bug: 348193756
Flag: EXEMPT minor refactoring

Change-Id: I9f53addc4b23e0e93179405e1a166adf8a3162a1