don't handle an amd module

[59naga]

currently v0.3.0-pre.2 is destroy amd modules.

const code = `export default 'default-entry';`
const result = babelTransform(code, {
  presets: [['env', {modules: 'amd'}]],
  plugins: [
    './src/index.js'
  ]
})
console.log(result.code)

becomes

define(['exports'], function (exports) {
  'use strict';

  Object.defineProperty(exports, "__esModule", {
    value: true
  });
  exports.default = 'default-entry';
  module.exports = exports.default; // "module" is not defined in browser
});

don't add module.exports.