pvmfw: use libavb_rs descriptor APIs

Now that libavb_rs provides the necessary descriptor APIs, stop parsing
them manually and use the library.

Bug: b/320542357
Test: atest libpvmfw_avb.integration_test
Change-Id: If272650150538e935844cc1cd3d091f85bdbb4cb
diff --git a/pvmfw/avb/src/lib.rs b/pvmfw/avb/src/lib.rs
index 9c3fe11..fd68652 100644
--- a/pvmfw/avb/src/lib.rs
+++ b/pvmfw/avb/src/lib.rs
@@ -18,13 +18,10 @@
 
 extern crate alloc;
 
-mod descriptor;
 mod error;
 mod ops;
 mod partition;
-mod utils;
 mod verify;
 
-pub use descriptor::Digest;
 pub use error::PvmfwVerifyError;
-pub use verify::{verify_payload, Capability, DebugLevel, VerifiedBootData};
+pub use verify::{verify_payload, Capability, DebugLevel, Digest, VerifiedBootData};