cmd/compile/internal/syntax: remove unused node field

cmd/compile/internal/syntax: remove unused node field

The doc field is not yet used - remove it for now (we may end up
with a different solution for 1.9). This reduces memory consumption
for parsing all of std lib by about 40MB and makes parsing slightly
faster.

Change-Id: Iafb00b9c7f1be9c66fdfb29096d3da5049b2fcf5
Reviewed-on: https://go-review.googlesource.com/33661
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>