lib/work: remove startup_lock

The startup_lock is garbage code now. The code of worker threads
is imported from tgt and the corresponding code in tgt is removed
in the following commit:
https://github.com/fujita/tgt/commit/c57e3ba1ae043eff558b4534e5bfe0c7865f6c1e
We should follow it.

Signed-off-by: Bingpeng Zhu <bingpeng.zbp@alibaba-inc.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>