I don't know much about modding Warband, so this might break something, but you can remove the background of the xp bar (not the green bar itself, though) by just changing the following lines in Modules/cRPG/meshes.txt
mesh_status_xp_bar 0 slider_hor 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 1.000000 1.000000 1.000000
mesh_status_xp_bar_button 0 handle_hor 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 1.000000 1.000000 1.000000
to
mesh_status_xp_bar 0 slider_hor 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000
mesh_status_xp_bar_button 0 handle_hor 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000