On-device signing: Remove Keymaster implementation.

This is no longer needed.

Bug: 165630556
Test: TEST_MAPPING
Change-Id: I5b5018d4f17827c515ffed5f188fd0ce1f34a08c
Merged-In: I5b5018d4f17827c515ffed5f188fd0ce1f34a08c
6 files changed