[dev.regabi] cmd/compile: remove fmt_test code; it has outlived its usefulness

With the recent compiler rewrites and cleanups to gc/fmt.go, the
"safety net" provided by fmt_test has become less important and
the test itself has become a burden (often breaks because of small
format changes elsewhere).

Eventually, the syntax and types2 packages will provide most error
and diagnostic compiler output at which point fmt.go can be further
simplified as well.

Change-Id: Ie93eefd3e1166f3548fed0199b732dbd6c81948a
Reviewed-on: https://go-review.googlesource.com/c/go/+/282560


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>