Remove gravatar_displaysize option, replace with user_picture_size

I propose that the "Image size" option in the Gravatar settings is unnecessary. We already have the 'user_picture_dimensions' variable which we should use instead of duplicating an option. The following code
would return a $size parameter, no larger than 512px (the maximum allowed by the gravatar service):

$size = min(explode('x', variable_get('user_picture_dimensions', '85x85')), 512);