Now that we have consistent use of xOrNil parse methods, we don't
need a special missing_statement singleton to distinguish between
missing actually statements and other errors (which have returned
nil before).

For #19663.

Change-Id: I8364f1441bdf8dd966bcd6d8219b2a42d6b88abd
Reviewed-on: https://go-review.googlesource.com/38656
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>