CGOPKGPATH variable was undocumented, but it is not needed anymore.

It was used before the existence of the go tool to tell cgo the full
path of the package that it was building, which in turn set the name
of the shared library that cgo expected to load back when cgo used
shared libraries. CGOPKGPATH no longer does anything useful;
it just affects the comments in the generated header file.

Remove it to avoid any future confusion.

Fixes #11852

Change-Id: Ieb452e5bbcfd05b87a4a3618b5b8f44423341858
Reviewed-on: https://go-review.googlesource.com/15266Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>