keymint: document deprecation of UNLOCKED_DEVICE_REQUIRED enforcement

KeyMint enforcement of UNLOCKED_DEVICE_REQUIRED is broken, has never
been used, and cannot be fixed.  So, document that it does not need to
be implemented.  Also remove the VTS test for it, which was disabled.

UNLOCKED_DEVICE_REQUIRED remains supported in Keystore.

Bug: 321100166
Test: Build
Change-Id: If4d47ee49c9d4a595820cfceb0f5f3027f99ee9f