Fixed compile warning on Mac caused by unused variables.

Summary:
Fixed compile warning caused by unused variables.

./db/compaction_picker.h:118:7: error: private field 'max_grandparent_overlap_factor_' is not used [-Werror,-Wunused-private-field]
  int max_grandparent_overlap_factor_;
      ^
./db/compaction_picker.h:119:7: error: private field 'expanded_compaction_factor_' is not used [-Werror,-Wunused-private-field]
  int expanded_compaction_factor_;
      ^
2 errors generated.

Test Plan:
make db_test