cmd/compile: remove unified IR quirks mode

Unified IR quirks mode existed to help bootstrap unified IR by forcing
it to produce bit-for-bit identical output to the original gc noder
and typechecker. However, I believe it's far enough along now to stand
on its own, plus we have good test coverage of generics already for
-G=3 mode.

Change-Id: I8bf412c8bb5d720eadeac3fe31f49dc73679da70
Reviewed-on: https://go-review.googlesource.com/c/go/+/385998
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

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

@@ -72,11 +72,7 @@ var localPkgReader *pkgReader
 func unified(noders []*noder) {
 	inline.NewInline = InlineCall
 
-	if !quirksMode() {
-		writeNewExportFunc = writeNewExport
-	} else if base.Flag.G != 0 {
-		base.Errorf("cannot use -G and -d=quirksmode together")
-	}
+	writeNewExportFunc = writeNewExport
 
 	newReadImportFunc = func(data string, pkg1 *types.Pkg, ctxt *types2.Context, packages map[string]*types2.Package) (pkg2 *types2.Package, err error) {
 		pr := newPkgDecoder(pkg1.Path, data)