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/33661Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/nodes.go

@@ -14,7 +14,8 @@ type Node interface {
 }
 
 type node struct {
-	doc  *Comment // nil means no comment(s) attached
+	// commented out for now since not yet used
+	// doc  *Comment // nil means no comment(s) attached
 	pos  uint32
 	line uint32
 }