roots: drop unnecessary mutex unlock in stop_request

In commit 31b2aa3c0dcf042559fa495b63a1bf22bac4d55d ("context: speed
up orderly process termination"), the stop request was split into two
methods after the mutex unlock.

Now that there's nothing after the mutex unlock in `stop_request`,
there's no need for an explicit unlock to do what the destructor would
have done anyway.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-roots.cc

@@ -1938,7 +1938,6 @@ BeesRoots::stop_request()
 	unique_lock<mutex> lock(m_stop_mutex);
 	m_stop_requested = true;
 	m_stop_condvar.notify_all();
-	lock.unlock();
 }
 
 void