Make each removal part conditional
    
    This also fixes an error where section removal was started at the wrong
    line (because we now remove some lines between the time the python2 package
    name was matched and we started the removal of sections). Let's just run
    the section removal over the whole file.
    
    Also, make it possible to generate the diff even if the result is not a
    valid spec file. This makes it easier to find out what is wrong. But don't
    write the broken file.