cmd/compile: remove unified IR stmtTypeDeclHack

After CL 385998, unified IR quirks mode was gone, it's time to remove
stmtTypeDeclHack, too.

Change-Id: Id73dd1d6c11b91c0c6c6cbe85f1b06977a9876d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/392214
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/noder/codes.go

@@ -29,9 +29,6 @@ const (
 	stmtFor
 	stmtSwitch
 	stmtSelect
-
-	// TODO(mdempsky): Remove after we don't care about toolstash -cmp.
-	stmtTypeDeclHack
 )
 
 type codeExpr int