cmd/compile/internal/gc: remove unneeded effects cache fields

Must have been lost when rebasing the SSA liveness CLs.

Change-Id: Iaac33158cc7c92ea44a023c242eb914a7d6979c6
Reviewed-on: https://go-review.googlesource.com/38427
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/plive.go

@@ -74,9 +74,6 @@ type progeffectscache struct {
 	textavarinit []int32
 	retuevar     []int32
 	tailuevar    []int32
-	uevar        [3]int32
-	varkill      [3]int32
-	avarinit     [3]int32
 	initialized  bool
 }