Revert "runtime: Goexit on C-created thread report more useful error message"

This reverts CL 602296.

Reason for revert: Failing on several builders.

Change-Id: I889c566d34294032c330d4f9402300ad0d5d3bf5
Reviewed-on: https://go-review.googlesource.com/c/go/+/611919
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/runtime/panic.go

@@ -614,8 +614,6 @@ func deferreturn() {
 // without func main returning. Since func main has not returned,
 // the program continues execution of other goroutines.
 // If all other goroutines exit, the program crashes.
-//
-// It crashes if called from a thread not created by the Go runtime.
 func Goexit() {
 	// Create a panic object for Goexit, so we can recognize when it might be
 	// bypassed by a recover().

src/runtime/proc.go

@@ -4321,9 +4321,6 @@ func gdestroy(gp *g) {
 
 	if locked && mp.lockedInt != 0 {
 		print("runtime: mp.lockedInt = ", mp.lockedInt, "\n")
-		if mp.isextra {
-			throw("runtime.Goexit called in a thread that was not created by the Go runtime")
-		}
 		throw("exited a goroutine internally locked to the OS thread")
 	}
 	gfput(pp, gp)

src/runtime/proc_test.go

@@ -1158,13 +1158,3 @@ func TestBigGOMAXPROCS(t *testing.T) {
 		t.Errorf("output:\n%s\nwanted:\nunknown function: NonexistentTest", output)
 	}
 }
-
-func TestCgoToGoCallGoexit(t *testing.T) {
-	if runtime.GOOS == "plan9" || runtime.GOOS == "windows" {
-		t.Skipf("no pthreads on %s", runtime.GOOS)
-	}
-	output := runTestProg(t, "testprogcgo", "CgoToGoCallGoexit")
-	if !strings.Contains(output, "runtime.Goexit called in a thread that was not created by the Go runtime") {
-		t.Fatalf("output should contain %s, got %s", "runtime.Goexit called in a thread that was not created by the Go runtime", output)
-	}
-}

src/runtime/testdata/testprogcgo/callback.go

@@ -38,21 +38,10 @@ import (
 
 func init() {
 	register("CgoCallbackGC", CgoCallbackGC)
-	register("CgoToGoCallGoexit", CgoToGoCallGoexit)
 }
 
-func CgoToGoCallGoexit() {
-	goexit = true
-	C.foo()
-}
-
-var goexit = false
-
 //export go_callback
 func go_callback() {
-	if goexit {
-		runtime.Goexit()
-	}
 	if e := extraMInUse.Load(); e == 0 {
 		fmt.Printf("in callback extraMInUse got %d want >0\n", e)
 		os.Exit(1)