Delete legacy backend tests, make SSA tests unconditional.

Next CL will remove _ssa from the file names.

Update #16357

Change-Id: I2a7f5dcbc69455f63b5e6e6b2725df26ab86c8dd
Reviewed-on: https://go-review.googlesource.com/29231
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>