Remove TODO to push down exec permission check

Pushing it down requires all implementation to check for
exec individualy which is not maintanable. Making it part
of GenericCheckPermissions add extra cost to everyone that
calls it. So it's better to keep is in
VirtualFilesystem.OpenAt.

Updates #1193

PiperOrigin-RevId: 302982993

pkg/sentry/vfs/vfs.go

@@ -402,8 +402,6 @@ func (vfs *VirtualFilesystem) OpenAt(ctx context.Context, creds *auth.Credential
 		if err == nil {
 			vfs.putResolvingPath(rp)
 
-			// TODO(gvisor.dev/issue/1193): Move inside fsimpl to avoid another call
-			// to FileDescription.Stat().
 			if opts.FileExec {
 				if fd.Mount().flags.NoExec {
 					fd.DecRef()