Remove the ast changes for async/await when ESLint supports it

[hzoo]

babel-eslint/babylon-to-espree/toAST.js

Lines 209 to 226 in aff95b8

	// async function as generator
	if (path.isFunction()) {
	if (node.async) node.generator = true;
	}
	// TODO: remove (old esprima)
	if (path.isFunction()) {
	if (!node.defaults) {
	node.defaults = [];
	}
	}
	// await transform to yield
	if (path.isAwaitExpression()) {
	node.type = "YieldExpression";
	node.delegate = node.all;
	delete node.all;
	}

    if (path.isFunction()) {
      if (node.async) node.generator = true;
    }

    // await transform to yield
    if (path.isAwaitExpression()) {
      node.type = "YieldExpression";
      node.delegate = node.all;
      delete node.all;
    }

Would need to modify the generator star spacing rule so we can also remove it in eslint-plugin-babel

[tomkel]

eslint 3.6.0 supports es2017 now

[hzoo]

Doing this in #361