IOThread: make io_thread_run() static

src/IOThread.cxx

@@ -35,7 +35,7 @@ static struct {
 	Thread thread;
 } io;
 
-void
+static void
 io_thread_run(void)
 {
 	assert(io_thread_inside());

src/IOThread.hxx

@@ -31,14 +31,6 @@ void
 io_thread_start();
 
 /**
- * Run the I/O event loop synchronously in the current thread.  This
- * can be called instead of io_thread_start().  For testing purposes
- * only.
- */
-void
-io_thread_run();
-
-/**
  * Ask the I/O thread to quit, but does not wait for it.  Usually, you
  * don't need to call this function, because io_thread_deinit()
  * includes this.