_sass: Remove outdated main-content-wrap override

Author: kevinlin1

_sass/custom/custom.scss

@@ -28,11 +28,6 @@ summary {
   width: 100%;
 }
 
-.main-content-wrap {
-  max-width: $content-width;
-  margin: auto;
-}
-
 .main-content {
   dl {
     display: block;