Fixed a compile error of cuckoo_table_builder.

Summary:
Fixed the following compile error.

./table/cuckoo_table_builder.h:72:22: error: private field 'key_length_' is not used [-Werror,-Wunused-private-field]
  const unsigned int key_length_;
                     ^
1 error generated.

Test Plan: make

Reviewers: sdong, ljin, radheshyamb, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20349

table/cuckoo_table_builder.cc

@@ -40,7 +40,6 @@ CuckooTableBuilder::CuckooTableBuilder(
       unsigned int))
     : num_hash_table_(std::min((unsigned int) 4, max_num_hash_table)),
       file_(file),
-      key_length_(fixed_key_length),
       value_length_(fixed_value_length),
       bucket_size_(fixed_key_length + fixed_value_length),
       hash_table_ratio_(hash_table_ratio),

table/cuckoo_table_builder.h

@@ -69,7 +69,6 @@ class CuckooTableBuilder: public TableBuilder {
 
   unsigned int num_hash_table_;
   WritableFile* file_;
-  const unsigned int key_length_;
   const unsigned int value_length_;
   const unsigned int bucket_size_;
   const double hash_table_ratio_;