deleting property in vm context has no effect

[deian]

• Version: 4.4.3, 5.10.1
• Platform: Arch Linux 4.5.0

Example failure:

const vm = require('vm');
const sbox = { };
vm.createContext(sbox);
vm.runInContext(`
  this.x = 'w00t';
  delete this.x;
`, sbox);

console.log(sbox); // { x: 'w00t' }

Expected sbox.x to be undefined.

[jasnell]

Related to #6283