remove CENTER_IMAGE

Hi,

It seems to me that using $CENTER_IMAGE to center all the images of an 
html document is a bad idea. In my opinion centered images should be 
centered in the texinfo document by using

@center @image{...}

And if a user wants to center all the images of the document he could still
override the default behaviour of &$image.

Are you ok for the removal of this variable ?

(Furthermore, the current implementation of @image, when $CENTER_IMAGE is 
true leads to invalid html.)

Pat