Remove ul capability

This capability indicates that underscore '_' overstrike current
letter under the cursor. It means that you can generate a
underline 'b' using 'b^H_', because it writes a 'b' then backward
one characther and then overstrike '_'. St has not such behaviour,
so it is an error to have this capability.

st.info

@@ -183,7 +183,6 @@ st| simpleterm,
 	smul=\E[4m,
 	tbc=\E[3g,
 	tsl=\E]0;,
-	ul,
 	xenl,
 	vpa=\E[%i%p1%dd,