Revert "Avoid a spandoc panic"

This reverts commit 71825eb.

Requires spandoc v0.2.1.
(Older spandoc versions will panic on this code.)

zebra-chain/src/work/difficulty/tests/vectors.rs

@@ -447,7 +447,7 @@ fn check_testnet_minimum_difficulty_block(height: block::Height) -> Result<(), R
             Some(spacing) if (time_gap <= spacing) => Err(eyre!(
                 "minimum difficulty block times must be more than 6 target spacing intervals apart"
             ))?,
-            _ => tracing::debug!("candidate minimum difficulty block {:?} {:?}", height, hash),
+            _ => {}
         };
     }