runtime: remove useless constant definition in malloc.go

runtime: remove useless constant definition in malloc.go

Change-Id: I060c867d89a06b5a44fbe77804c19299385802d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/311250
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>