Removed unneeded sqlite3_threadsafe() check

Since the database is accessed from 1 thread only
thanks to `key` parameter of `driver_async`.
In fact we could previously potentially access the
same DB twice from different emulator threads
precisely when SQLite3 is compiled in single-thread
mode.

c_src/sqlite3_drv.c

@@ -423,13 +423,8 @@ static inline async_sqlite3_command *make_async_command_script(
 static inline void exec_async_command(
     sqlite3_drv_t *drv, void (*async_invoke)(void*),
     async_sqlite3_command *async_command) {
-  if (sqlite3_threadsafe()) {
   drv->async_handle = driver_async(drv->port, &drv->key, async_invoke,
                                    async_command, sql_free_async);
-  } else {
-    async_invoke(async_command);
-    ready_async((ErlDrvData) drv, (ErlDrvThreadData) async_command);
-  }  
 }
 
 static inline int sql_exec_statement(