ovl: remove redundant IOCB_DIO_CALLER_COMP clearing

The backing_file_write_iter() function, which is called
immediately after this code, already contains identical
logic to clear the IOCB_DIO_CALLER_COMP flag along with
the same explanatory comment. There is no need to duplicate
this operation in the overlayfs code.

Signed-off-by: Seong-Gwang Heo <heo@mykernel.net>
Fixes: a6293b3e ("fs: factor out backing_file_{read,write}_iter() helpers")
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>