The site is not loading at the moment and returning a 504 Gateway Time-out error.
Regarding the parallax-slider.less (or parallax-slider.css) file, it is most likely added by a plugin to the page.
Even though you do not have the plugin content (in this case, a slider) on the page, some files from this plugin may still be added to each page. This is often the case when plugins work with shortcodes. Parsing the complete text of each page to see if it contains a shortcode or not is a very consuming process and it is usually not recommended unless necessary. That is why, many plugins simply add their files without checking if their shortcode is used on the page or not.
WordPress does not have a more efficient way (yet) of figuring out what pages have what shortcodes and then to load only the necessary files for them. Hopefully, in the future, this will be improved.
So, it is enough for the plugin to be activated and it may add its files to each page.
You should go look through your plugins and see which plugin may be adding this particular file. And you should also deactivate the plugins that you are not using to be sure that they do not add any unnecessary code and affect the speed of the site.