Don't mark async functions as hoist

`babel-template` no longer crashes in IE attempting to access unpopulated error stack

packages/babel-helper-remap-async-to-generator/src/index.js

@@ -70,7 +70,6 @@ function plainFunction(path: NodePath, callId: Object) {
         t.callExpression(container, [])
       )
     ]);
-    declar._blockHoist = true;
 
     retFunction.id = asyncFnId;
     path.replaceWith(declar);

packages/babel-plugin-transform-async-to-generator/test/fixtures/async-to-generator/statement/actual.js

@@ -1,3 +1,5 @@
+function normalFunction() {}
+
 async function foo() {
   var wat = await bar();
 }

packages/babel-plugin-transform-async-to-generator/test/fixtures/async-to-generator/statement/expected.js

@@ -1,3 +1,5 @@
+function normalFunction() {}
+
 let foo = (function () {
   var ref = babelHelpers.asyncToGenerator(function* () {
     var wat = yield bar();