engines/splice: remove buggy ->mem_align set

engines/splice: remove buggy ->mem_align set

Two things wrong here:

1) We align buffers by default, so no need for splice to do anything
   extra.
2) ->mem_align is not a true/false setting, it's the alignment itself.
   Hence the current setting to 1 is just buggy.

Fixes: https://github.com/axboe/fio/issues/810
Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>