remove custom.css since there's already a site-css setting

Author: daattali

_layouts/base.html

@@ -2,7 +2,6 @@
 common-css:
   - "/assets/css/bootstrap-social.css"
   - "/assets/css/beautifuljekyll.css"
-  - "/assets/css/custom.css"
 common-ext-css:
   - href: "https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css"
     sri: "sha384-Vkoo8x4CGsO3+Hhxv8T/Q5PaXtkKtu6ug5TOeNV6gBiFeWPGFN9MuhOf23Q9Ifjh"