do not delete this.options.html

I have a setup where I am processing two trees and the `html` option is the same for both:

    var uncssOptions = { html: ['./index.html'] };
    vendorCss = uncss(vendorCss, uncssOptions);
    appCss = uncss(appCss, uncssOptions);

This causes an error:

    throw new Error('`html` option required');

So, to be able to share a single options object I removed the delete statement.