remove spurious file

assets/main.scss

----
-# Only the main Sass file needs front matter (the dashes are enough)
----
-
-@charset "utf-8";
-
-@import "minimal-mistakes/skins/{{ site.minimal_mistakes_skin | default: 'default' }}"; // skin
-@import "minimal-mistakes"; // main partials
-
-@import "dynare"