These used to be necessary when racing with updates to the mark bit,
but since the mark bit is no longer in the bitmap and the checkmark is
only updated with the world stopped, we can now always use regular
writes to update the type information in the heap bitmap.

Somewhat surprisingly, this has basically no overall performance
effect beyond the usual noise, but it does clean up the code.

Change-Id: I3933d0b4c0bc1c9bcf6313613515c0b496212105
Reviewed-on: https://go-review.googlesource.com/29277
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>