HTMLMODE_FULL_ABS_POS appears to have been used for enabling accurate
margin settings for absolutely positioned graphics objects. If it was
disabled one could not set (in HTML editor mode) separate margins for
left and right (or bottom/top) sides of the image. The setting was
disabled for Netscape and enabled for other browsers.

This patch removes the setting and unconditionally enables the feature
in HTML editor mode. All modern browsers support
margin-{left,right,bottom,top} CSS properties.