Use a single Substrate signer, per intentions in #227

Removes key from Update as well, since it's no longer variable.

processor/messages/src/lib.rs

@@ -176,7 +176,7 @@ pub mod substrate {
 
   #[derive(Clone, PartialEq, Eq, Debug, Zeroize, Serialize, Deserialize)]
   pub enum ProcessorMessage {
-    Update { key: [u8; 32], batch: SignedBatch },
+    Update { batch: SignedBatch },
   }
 }