Update_verifier: Remove the support for legacy text format CareMap

We have already switched to the protobuf format for new builds, and
the downgrade packages will require a data wipe. So it should be safe
to drop the support for text format.

This also helps to save the issue when users sideload a package with a
pending OTA, because the new CareMap contains the fingerprint of the
intended build.

Bug: 128536706
Test: unit tests pass, run update_verifier with legacy CareMap
Change-Id: I1c4d0e54ec591f16cc0a65dac76767725ff9e7c4
(cherry picked from commit aaa6103a)