Revert "Fix work challenge lockscreen being dismissed"

This should no longer be necessary because
ConfirmDeviceCredentialActivity does not finish when it's paused, so the
special-case handling for when the work profile is locked is no longer
needed.

This reverts commit 145d69a4.

Bug: 153689182
Bug: 141470517
Test: Manual, set up a work profile and double-tap a work notification
or try to Reply to a work GMail notification.

Change-Id: I528deebb54f6d6f229d175b864ee5427cf4253a5