Remove booting from kernels in raw/qcow2 images

Remove booting from kernels in raw/qcow2 images

Diff and (slightly tweaked) text below from
Dave Voutila < dave at sisu dot io >, thanks!

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Since 6.7 switched to FFS2 as the default filesystem for new installs,
the ability for vmd(8) to load a kernel and boot.conf from a disk image
directly (without SeaBIOS) has been broken.

A diff from tb to add FFS2 support never mdae it into the tree.

On 5th Jan 2021, new ramdisks for amd64 have started shipping gzipped,
breaking the ability to load the bsd.rd directly as a kernel image for a vmd
guest without first uncompressing the image.

Using BIOS works, the FFS2 change happend ten months ago and few if any have
complained about the breakage.  vmctl(8) is still vague about supporting it
per its man page and one still has to pass the disk image twice as a "-b"
and "-d" argument to boot an OpenBSD guest *without* BIOS.

Josh Rickmar reported the gzip issue on bugs@ and provided patches to add
support for compressed ramdisks and kernel images.  The easiest way to do so
is to drop support for FFS images since they require a call to fmemopen(3)
while all the other logic uses fopen(3)/fdopen(3) calls and a file
descriptor.  It is much easier to get thsoe patches merged if they don't
have to account for extracting files from disk images.
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No objections anyone
"Removing it makes sense" reyk (who wrote the FFS module)
OK mlarkin