Dropping octave's native image format

[Carnë Draug]

Hi

one thing that I never knew about was Octave's native image format.
Basically, this "image format" is any file that can be read with
load() and contains 1 or 2 variables, depending if it's an indexed
image. If it's an indexed image, the two variables must be named "X"
and "map" (the indexed image, and the colormap) otherwise it must have
a variable named "img".

This is what I figured by reading the source of imread. As far as can
see, this format is completely undocumented, and we don't even have a
function to write into it. Should we remove this? I'm guessing that
even if we removed it, no one would notice (except for when calling
image() without arguments since it loads default.img, the only example
I have of an image in this format).

Carnë