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

Given that 849d18e ("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