runtime: don't print EnableGC flag in WriteHeapProfile()

runtime: don't print EnableGC flag in WriteHeapProfile()

Current runtime.WriteHeapProfile() doesn't print correct
EnableGC. Even if GOGC=off, the result file has below line:
 # EnableGC = true

It is hard to print correct status of the variable because of corner
cases e.g. initialization. For avoiding confusion, this commit removes
the print.

Change-Id: Ia792454a6c650bdc50a06fbaff4df7b6330ae08a
Reviewed-on: https://go-review.googlesource.com/18600
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>