aio: remove an outdated BUG_ON and comment in aio_complete

These days we don't treat sync iocbs special in the aio completion code as
they never use it.  Remove the old comment and BUG_ON given that the
current definition of is_sync_kiocb makes it impossible to hit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Change-Id: I67331a81f721c12072f51cd7d3e8bd1ce3f56966
Git-commit: c213dc826c591cec9e1a458f17ae474af6c8e00c
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Jack Pham <jackp@codeaurora.org>