remove unnecessary compaction condition check for key below watermark (#145)

remove unnecessary condition check

mini-lsm-mvcc/src/compact.rs

@@ -161,7 +161,7 @@ impl LsmStorageInner {
             }
 
             if iter.key().ts() <= watermark {
-                if same_as_last_key && !first_key_below_watermark {
+                if !first_key_below_watermark {
                     iter.next()?;
                     continue;
                 }