Clean up `DesktopTasksControllerTest`

Remove unnecessary mocking of
`DesktopModeStatus.enforceDeviceRestrictions` and
`DesktopModeStatus.isDesktopModeSupported`.

Remove tests which attempt to modify
`DesktopModeStatus.canEnterDesktopMode` through mocking as these have no
affect as `canEnterDesktopMode` is only used in the initialisation of
the `DesktopTasksController` which happens in the setup before the test.

Fixes: 346785442
Test: atest WMShellUnitTests:DesktopTasksControllerTest
Change-Id: I2a6a87bb74d5588cbab9bd24e9e0a261913a9437