Add an experimental multi-threaded capability to vdbesorter.c.

FossilOrigin-Name: ff0b5c851ba7d04d1836d7c6a3222713e7d8d357

src/threads.c

@@ -47,7 +47,6 @@ int sqlite3ThreadCreate(
   void *pIn                 /* Argument passed into xTask() */
 ){
   SQLiteThread *p;
-  int rc;
 
   assert( ppThread!=0 );
   assert( xTask!=0 );