Remove unlock_user_key function

Keystore2 super key handling is being refactored in preparation for
Unlocked-Only Storage.

This function is dead code. It has no callers.

Bug: 280502317
Bug: 277798192
Test: Wiped device. Setup user with PIN. Ensured unlock works. Remove
    PIN. Ensured unlock works. Added pin and biometric. Ensured unlock
    works.  Rebooted device. Ensured unlock works.
Change-Id: I4c7791f6944afb621afb2d67f4b7b7d4690ddd78
1 file changed