Fix warning -Wextra-semi. NFC. (#6869)

Summary:
Minor fix.

CLA signed.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6869

Reviewed By: ajkr

Differential Revision: D21704001

Pulled By: pdillinger

fbshipit-source-id: 57fd08114f3234f51f34758e25e708cc70962582

utilities/cassandra/format.h

@@ -157,7 +157,7 @@ public:
   RowValue& operator=(const RowValue& /*that*/) = delete;
   RowValue& operator=(RowValue&& /*that*/) = default;
 
-  std::size_t Size() const;;
+  std::size_t Size() const;
   bool IsTombstone() const;
   // For Tombstone this returns the marked_for_delete_at_,
   // otherwise it returns the max timestamp of containing columns.