Remove unused variable (#8067)

Summary:
Remove unused variable `Slice blob_to_write` in `db/blob/blob_file_cache_test.cc`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8067

Reviewed By: zhichao-cao

Differential Revision: D27107693

Pulled By: riversand963

fbshipit-source-id: 9bfd4d296a6a1714ad5c1fa5bb231a0c52dbd56d

db/blob/blob_file_cache_test.cc

@@ -64,7 +64,6 @@ void WriteBlobFile(uint32_t column_family_id,
   constexpr char blob[] = "blob";
 
   std::string compressed_blob;
-  Slice blob_to_write;
 
   uint64_t key_offset = 0;
   uint64_t blob_offset = 0;