[tlbuild] pruning platforms

[Karl Berry]

    mipsel-linux please

Good, that's a big one :).

    I'm guessing that many arm* platforms don't even have a DVD unit.

Good point, I hadn't thought of that.  So I may as well omit armel-linux
and armhf-linux too, I suppose.  (I realize that in theory people could
want to read the DVD on some other system and install for arm and then
copy, or some such, but ... seems pretty rare.  Such people should be
able to deal.)

Thanks,
Karl