Remove /etc/apt/apt.conf.d/01autoremove-kernels in mkimage/debootstrap

This file is one APT creates to make sure we don't "autoremove" our currently in-use kernel, which doesn't really apply to debootstraps/Docker images that don't even have kernels installed.

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>
Merge pull request #9822 from tianon/01autoremove-kernels

Remove /etc/apt/apt.conf.d/01autoremove-kernels in mkimage/debootstrap