cmd/compile: remove -G flag

Post 1.18, we're committed to types2 as cmd/compile's type checker.

Change-Id: I30d2dd2b2ba62832fcdcaeb996fcbc8a4a05d591
Reviewed-on: https://go-review.googlesource.com/c/go/+/388535
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -6,7 +6,6 @@ package noder
 
 import (
 	"fmt"
-	"os"
 
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/dwarfgen"
@@ -77,10 +76,6 @@ func checkFiles(noders []*noder) (posMap, *types2.Package, *types2.Info) {
 func check2(noders []*noder) {
 	m, pkg, info := checkFiles(noders)
 
-	if base.Flag.G < 2 {
-		os.Exit(0)
-	}
-
 	g := irgen{
 		target: typecheck.Target,
 		self:   pkg,
@@ -90,10 +85,6 @@ func check2(noders []*noder) {
 		typs:   make(map[types2.Type]*types.Type),
 	}
 	g.generate(noders)
-
-	if base.Flag.G < 3 {
-		os.Exit(0)
-	}
 }
 
 // Information about sub-dictionary entries in a dictionary