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/base/debug.go

@@ -39,7 +39,6 @@ type DebugFlags struct {
 	TypeAssert           int    `help:"print information about type assertion inlining"`
 	TypecheckInl         int    `help:"eager typechecking of inline function bodies"`
 	Unified              int    `help:"enable unified IR construction"`
-	UnifiedQuirks        int    `help:"enable unified IR construction's quirks mode"`
 	WB                   int    `help:"print information about write barriers"`
 	ABIWrap              int    `help:"print information about ABI wrapper generation"`
 	MayMoreStack         string `help:"call named function before all stack growth checks"`