cmd/yacc: fix default action

Previously, a production rule like

	A: B C D

would cause yacc to check that A and B have the same declared types,
but then it would generate an implicit action of { $$ = $3 } (i.e.,
copy the value from D), even if A and D have different types.

Fixes #10192.

Change-Id: I51cfd7baa0011557141dca33b7af1d892cc6f49e
Reviewed-on: https://go-review.googlesource.com/7780
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/yacc/yacc.go

@@ -637,9 +637,6 @@ outer:
 			if tempty != nontrst[curprod[0]-NTBASE].value {
 				lerrorf(ruleline, "default action causes potential type clash")
 			}
-			fmt.Fprintf(fcode, "\n\tcase %v:", nprod)
-			fmt.Fprintf(fcode, "\n\t\t%sVAL.%v = %sS[%spt-0].%v",
-				prefix, typeset[tempty], prefix, prefix, typeset[tempty])
 		}
 		moreprod()
 		prdptr[nprod] = make([]int, mem)