Remove useless counter Innodb_buffered_aio_submitted

MDEV-18582 ported many performance counters from XtraDB to the InnoDB implementation in MariaDB Server. Among them was buffered_aio_submitted or Innodb_buffered_aio_submitted, whose value should always be 0 or 1, depending on whether is_linux_native_aio_supported() was executed to the point where it would be incremented.

Let us remove this counter, because it has no practical value. Even if its value were 1, io_setup() can still fail and we may have innodb_use_native_aio=0.