ash: trivial code shrink

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/ash.c b/shell/ash.c
index 99fdbce..18ccc13 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -10018,7 +10018,7 @@
 				setvareq(name, flags);
 			else
 				/* "local VAR" unsets VAR: */
-				setvar0(name, NULL);
+				unsetvar(name);
 		}
 	}
 	lvp->vp = vp;