cmd/compile/internal/gc: remove unparen

CL 197120 removed the last use of it.

Change-Id: I5fe4f57a47acc712208d831e72cd79205a534c28
Reviewed-on: https://go-review.googlesource.com/c/go/+/252697
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/cmd/compile/internal/gc/noder.go b/src/cmd/compile/internal/gc/noder.go
index 590c1a1..5dce533 100644
--- a/src/cmd/compile/internal/gc/noder.go
+++ b/src/cmd/compile/internal/gc/noder.go
@@ -1641,10 +1641,3 @@
 	}
 	return n
 }
-
-func unparen(x *Node) *Node {
-	for x.Op == OPAREN {
-		x = x.Left
-	}
-	return x
-}