cross: Drop unused do_install

All the users of the cross class I could find redefine the do_install
function. It is unusual and likely undesirable for the class to be
changing this from the default and likely a leftover from different
times. Remove it as it isn't doing anything.

[YOCTO #9272]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>