cmd/compile: remove stray blank assignment

Left over from CL 39855.

Change-Id: I9df8b5c631d5afbdbf2fb306876648d8541931d3
Reviewed-on: https://go-review.googlesource.com/40941
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -192,7 +192,6 @@ func autotmpname(n int) string {
 	// Start with a buffer big enough to hold a large n.
 	b := []byte(prefix + "      ")[:len(prefix)]
 	b = strconv.AppendInt(b, int64(n), 10)
-	_ = b
 	return types.InternString(b)
 }