Revert "thread: Judge joinable before join"

To fix:
thread.cpp: In destructor ‘virtual Thread::~Thread()’:
thread.cpp:71:19: error: ‘class NativeThread’ has no member named ‘joinable’
   71 |     if (stdThread.joinable()) {
      |                   ^~~~~~~~
make: *** [<builtin>: thread.o] Error 1

This reverts commit 2dfee5c.

Change-Id: I528404357c26d1781c3b2d9673ea6f60be38a995

src/thread.cpp

@@ -68,9 +68,7 @@ Thread::~Thread()
 
     exit = true;
     start_searching();
-    if (stdThread.joinable()) {
-        stdThread.join();
-    }
+    stdThread.join();
 }
 
 /// Thread::clear() reset histories, usually before a new game