Remove salt from MM's instance.img partition

It was deprecated in Android 15 with llpvm_changes & replaced with
instance-id based differentiation of secrets for Nonprotected VM. It
was only around to support cases when llpvm_changes was not set.

Now that we no longer need/depend on this flag, remove it from
Microdroid Manager.

Test: Builds
Bug: 383553863
Change-Id: I2f8b966518b3b4b47718d18c2881eadea23c375e
diff --git a/guest/microdroid_manager/src/dice.rs b/guest/microdroid_manager/src/dice.rs
index 7cfeb21..edc4d63 100644
--- a/guest/microdroid_manager/src/dice.rs
+++ b/guest/microdroid_manager/src/dice.rs
@@ -53,11 +53,7 @@
     let debuggable = is_debuggable()?;
 
     // Send the details to diced
-    let hidden = if cfg!(llpvm_changes) {
-        hidden_input_from_instance_id()?
-    } else {
-        instance_data.salt.clone().try_into().unwrap()
-    };
+    let hidden = hidden_input_from_instance_id()?;
     dice.derive(code_hash, &config_descriptor, authority_hash, debuggable, hidden)
 }