In commit 412533b4 (MDEV-18582),
one of the counters that was ported from XtraDB is useless.
Innodb_buffered_aio_submitted would 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
end up with innodb_use_native_aio=0.