remove the `unhandledRejection` custom handler from the setup script

[travi]

mocha-scaffolder/templates/mocha-setup.txt

Lines 6 to 8 in 32f2c5e

	process.on('unhandledRejection', reason => {
	throw reason;
	});

In Mocha v8.2.0 we added an unhandledRejection listener.

mochajs/mocha#2640 (comment)