fmt: hold on to all free buffers, regardless of size

fmt: hold on to all free buffers, regardless of size

This code made sense before fmt switched to using sync.Pool, but a
sync.Pool clears all items on GC, so not reusing something based on
size is just a waste of memory.

Change-Id: I201312b0ee6c572ff3c0ffaf71e42623a160d23f
Reviewed-on: https://go-review.googlesource.com/21480
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>