Incorrect print of default export of template literal

When printing a default export whose value is or includes a template literal, there are extra backticks in the output:

```js
const recast = require("recast");
const types = require("ast-types");

const ast = recast.parse("export default `${3}`");

types.visit(ast, {
visitTemplateLiteral(path) {
console.log(recast.print(path.node).code);
return false;
}
});
```

## Actual behavior

The above example code logs '``${3}``', which isn't valid JS

## Expected behavior

The above example code should log '`${3}`'

## Details

Tested with Recast 0.18.1 (latest as of this writing)

Thanks for your help in figuring this out!

Deleting the two calls to 'parts.push("\`")' in the `case "TemplateLiteral"` statement in printer.ts seems like it might fix the issue.

Start by reproducing the default-export template-literal example from the issue with Recast, then inspect the `case "TemplateLiteral"` statement in `printer.ts`, especially the two `parts.push("\`")` calls. Done means `recast.print(path.node).code` produces ```${3}``` rather than extra backticks and the example output is valid JavaScript.

Written by the indexing model from the issue text.