metacity-theme-3.xml

<!-- /usr/share/themes/elementary/metacity-1/metacity-theme-3.xml -->
<frame_geometry name="max" has_title="false" title_scale="medium" parent="normal" rounded_top_left="false" rounded_top_right="false">
         <distance name="left_width" value="0" />
         <distance name="right_width" value="0" />
         <distance name="left_titlebar_edge" value="0"/>
         <distance name="right_titlebar_edge" value="0"/>
         <distance name="title_vertical_pad" value="0"/>
         <border name="title_border" left="0" right="0" top="0" bottom="0"/>
         <border name="button_border" left="0" right="0" top="0" bottom="0"/>
         <distance name="bottom_height" value="0" />
</frame_geometry>

Remove borders on fullscreen.