The old whole-page reclaimer was the only thing that used the busy
span lists. Remove them so nothing uses them any more.

Change-Id: I4007dd2be08b9ef41bfdb0c387215c73c392cc4c
Reviewed-on: https://go-review.googlesource.com/c/138960
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>