md/linear: Get rid of md-linear.h

Given that 849d18e2 ("md: Remove deprecated CONFIG_MD_LINEAR")
killed the linear flavour of MD, it seems only logical to drop
the leftover include file that used to come with it.

I also feel that it should be my own privilege to remove my 30 year
old attempt at writing kernel code ;-). RIP!

Cc: Song Liu <song@kernel.org>
Cc: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240201224549.750644-1-maz@kernel.org

drivers/md/md-linear.h

-/* SPDX-License-Identifier: GPL-2.0 */
-#ifndef _LINEAR_H
-#define _LINEAR_H
-
-struct dev_info {
-	struct md_rdev	*rdev;
-	sector_t	end_sector;
-};
-
-struct linear_conf
-{
-	struct rcu_head		rcu;
-	sector_t		array_sectors;
-	int			raid_disks; /* a copy of mddev->raid_disks */
-	struct dev_info		disks[] __counted_by(raid_disks);
-};
-#endif