to unify signature with pthread's one

I'm planning to use functions for rb_nativethread_cond_t and
rb_nativethread_mutex_t in the future JIT introduction.

In that case, I want them to have the same signature. To prevent the case
that its return value is used in somewhere and it becomes harder to unify
signature, I want to drop unused return value.

[close GH-1751]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60811 b2dd03c8-39d4-4d8f-98ff-823fe69b080e