No longer need to cancel authentication from ConfirmDeviceCredentialActivity

CDCA no longer needs to cancel authentication in onPause. Since it
internally invokes BiometricPrompt, and BiometricPrompt's components e.g.
BiometricService and AuthController are aware of the "top-ness" of its
client, this code is redundant.

Fixes: 145991060

Test: Follow comment#3 in the bug above, repeat 10+ times
Test: Set up work profile, set up work profile password. Open work profile
      app, but before entering password, swipe up to go to home screen.
      Authentication is cancelled as expected.

Change-Id: I0b4d7d89cb9801ddbb6e3bd07f71191035cc75ec