The lastPos field used in the past to track the line number of a token.
it's irrelevant anymore, and we can remove it.

Change-Id: I42c0bf55e884b79574a7da4926489f2d77618cd0
Reviewed-on: https://go-review.googlesource.com/49591Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>