This CL removes an outdated comment regarding converting a pointer to `uintptr`.
The comment was introduced in Go 1.4 and runtime GC was under the consideration of major revisions. According to the current situation, Go runtime memory allocator has no fragmentation issue. Therefore compact GC won't be implemented in the near future.

Change-Id: I5c336d81d810cf57b76797f05428421bb39a5b9f
GitHub-Last-Rev: 2ab4be3885d3f48abbcb59af3f74bc95501ff23f
GitHub-Pull-Request: golang/go#33685
Reviewed-on: https://go-review.googlesource.com/c/go/+/190520Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>