mheap_.treapalloc.alloc() already returns a zeroed treapNode. Don't
bother re-zeroing all of the fields.

Change-Id: Iea317040fbb72dfe5ef1e2c56c287680b065f2d9
Reviewed-on: https://go-review.googlesource.com/c/139460
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>