refactor: remove manual Debug impl

Derive the debug impl so it prints all the fields (specifically the
"number of sequencers configured" is pretty helpful in a test).

Manual impls drift over time and are more effort than the derive!

write_buffer/src/mock.rs

@@ -347,6 +347,7 @@ impl WriteBufferWriting for MockBufferForWritingThatAlwaysErrors {
     }
 }
 
+#[derive(Debug)]
 pub struct MockBufferForReading {
     shared_state: MockBufferSharedState,
     n_sequencers: u32,
@@ -377,12 +378,6 @@ impl MockBufferForReading {
     }
 }
 
-impl std::fmt::Debug for MockBufferForReading {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        f.debug_struct("MockBufferForReading").finish()
-    }
-}
-
 /// Sequencer-specific playback state
 #[derive(Debug)]
 pub struct MockBufferStreamHandler {