We already have the ptrdata field in a type, which encodes exactly
the same information that kindNoPointers does.

My problem with kindNoPointers is that it often leads to
double-negative code like:

   t.kind & kindNoPointers != 0

Much clearer is:

   t.ptrdata == 0

Update #27167

Change-Id: I92307d7f018a6bbe3daca4a4abb4225e359349b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/169157
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>