Remove duplicate *.onum glyphs

I added these back in 9c764b6dc6e020aedc3373561a09f41f479847e3. But
upon further reflection they're basically pointless, the normal glyphs
can be used in features instead. More significantly, they introduce
further hinted/unhinted incompatibility and so should be removed if
possible.

res/diacritics.txt

@@ -555,10 +555,6 @@ eight=eight.pnum/20,20
 nine=nine.pnum/0,20
 zero=zero.pnum/20,20
 #
-# old-style numbers
-six=six.onum
-eight=eight.onum
-#
 # AddGlyphs
 uni1FBE=uni037A
 #I=Iota1

src/v2/Roboto_Regular.ufo/features.fea

@@ -34,7 +34,7 @@ languagesystem grek dflt;
 @SC_SET5 = [ Mu.smcp uni0401.smcp uni0402.smcp uni0403.smcp uni0404.smcp uni0405.smcp uni0406.smcp uni0407.smcp uni0408.smcp uni0409.smcp uni040A.smcp uni040B.smcp uni040C.smcp uni040E.smcp uni040F.smcp ];
 
 @LNUM = [zero one two three four five six seven eight nine];
-@ONUM = [zero.onum one.onum two.onum three.onum four.onum five.onum six.onum seven.onum eight.onum nine.onum];
+@ONUM = [zero.onum one.onum two.onum three.onum four.onum five.onum six seven.onum eight nine.onum];
 @PNUM = [zero.pnum one.pnum two.pnum three.pnum four.pnum five.pnum six.pnum seven.pnum eight.pnum nine.pnum];
 @TNUM = [zero.tnum one.tnum two.tnum three.tnum four.tnum five.tnum six.tnum seven.tnum eight.tnum nine.tnum];
 @frac1 = [ zero one two three four five six seven eight nine ];