runtime: remove use of obsolete map deletion syntax

The use of this syntax was eliminated upstream in Go 992248b2adc2,
but this particular use slipped through somehow.

From-SVN: r209506

libgo/go/syscall/syscall_unix.go

@@ -153,7 +153,7 @@ func (m *mmapper) Munmap(data []byte) (err error) {
 	if errno := m.munmap(uintptr(unsafe.Pointer(&b[0])), uintptr(len(b))); errno != nil {
 		return errno
 	}
-	m.active[p] = nil, false
+	delete(m.active, p)
 	return nil
 }