ondevice-signing: remove unused cert and signature code

Remove code that became unused due to the following commits:

- https://r.android.com/1988348
  ("Switch from compos_verify_key to compos_verify").

- https://r.android.com/2362310
  ("Stop adding cert of early boot key to fs-verity keyring")

Bug: 290064770
Test: atest odsign_e2e_tests_full
Change-Id: I50643f5ddf570d7185c577ec1d6bb8cd81c7af45
4 files changed