Revert "io: allocate copy buffers from a pool"

Revert "io: allocate copy buffers from a pool"

This reverts CL 456555.

Reason for revert: This seems too likely to exercise race conditions
in code where a Write call continues to access its buffer after
returning. The HTTP/2 ResponseWriter is one such example.

Reverting this change while we think about this some more.

For #57202

Change-Id: Ic86823f81d7da410ea6b3f17fb5b3f9a979e3340
Reviewed-on: https://go-review.googlesource.com/c/go/+/467095
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>