tests: Remove rootfs from kernel config test

The rootfs fs was removed in 5.3.x but was mostly a
irrelevant entry in the filesystems list anyway.

Here is the upstream commit:

commit fd3e007f6c6a0f677e4ee8aca4b9bab8ad6cab9a
Author: Al Viro <viro@zeniv.linux.org.uk>
Date:   Thu May 30 17:48:35 2019 -0400

    don't bother with registering rootfs

    init_mount_tree() can get to rootfs_fs_type directly and that simplifies
    a lot of things.  We don't need to register it, we don't need to look
    it up *and* we don't need to bother with preventing subsequent userland
    mounts.  That's the way we should've done that from the very beginning.

    There is a user-visible change, namely the disappearance of "rootfs"
    from /proc/filesystems.  Note that it's been unmountable all along
    and it didn't show up in /proc/mounts; however, it *is* a user-visible
    change and theoretically some script might've been using its presence
    in /proc/filesystems to tell 2.4.11+ from earlier kernels.

    *IF* any complaints about behaviour change do show up, we could fake
    it in /proc/filesystems.  I very much doubt we'll have to, though.

    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Signed-off-by: Rolf Neugebauer <rn@rneugeba.io>

test/pkg/kernel-config/check-kernel-config.sh

@@ -130,7 +130,6 @@ done
 # check filesystems that are built in
 for fs in \
 sysfs \
-rootfs \
 tmpfs \
 bdev \
 proc \