ssaMain is no longer needed.

Change-Id: I0b77f0bcd482329d73018bd80a6e068e622e191b
Reviewed-on: https://go-review.googlesource.com/27190Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>