Remove unused eligibility flag for Perceptible Tasks

Eligibility of perceptible tasks is no longer configured per device. It
is now determined based off of system capabilities (supports PC mode or
freeform windows) - "desktop mode".

Bug: 397076790
Test: Manual
Flag: com.android.server.am.perceptible_tasks
Change-Id: I13be1a725512beb4e86fb1ca3f609f850bc65a65