Fix unused variable warnings in db_bench_tool.cc

[git-hulk]

num_seek_to_first and num_next are not used in db_bench_tool. It will throw warnings and then fail to build.

/Users/hulk/code/cxx/speedb/tools/db_bench_tool.cc:7693:14: error: variable 'num_seek_to_first' set but not used [-Werror,-Wunused-but-set-variable]
    uint64_t num_seek_to_first = 0;
             ^
/Users/hulk/code/cxx/speedb/tools/db_bench_tool.cc:7694:14: error: variable 'num_next' set but not used [-Werror,-Wunused-but-set-variable]
    uint64_t num_next = 0;

[Yuval-Ariel]

LGTM thanks a lot!