Remove nodemailer configuration from Grunt script

Unlike the original [boilerplate](https://github.com/dwightjack/grunt-email-boilerplate/blob/master/Gruntfile.js#L255), nodemailer options are being added directly to the [Grunt configuration file](https://github.com/jahvi/generator-htmlemail/blob/master/app/templates/_Gruntfile.js#L150). This [change](https://github.com/matbrady/generator-htmlemail/commit/c01c0b23cfe4afcb2d3cd673ed713a47d26010d3) lessens the risk of accidentally publishing account credentials if the project is made public.