Guess that Nom+Acc become Gen in negative, remove neggen field

This removes around 70k concrete categories (in PGFs compiled with
--optimize-pgf.)
Unsure if that is actually useful -- the number of concrete functions
only went down by a couple hundred.

src/russian/VerbRus.gf

@@ -114,7 +114,7 @@ lin
         PrefShort => ap.short
         } ;
       Neg => case ap.preferShort of {
-        PreferFull => case v2a.c.neggen of {
+        PreferFull => case neggen v2a.c of {
             False => \\a => ap.s ! agrGenNum a ! Animate ! v2a.c.c ;
             True => \\a => ap.s ! agrGenNum a ! Animate ! Gen
           } ;