#content overflows parent container in fixed width layout with no sidebars

Width of #content with no sidebars overflows its parent container (#main). This is due to both #main and #content having their widths set to the same value ($zen-grid-width) while not accounting for #content having margins based on $zen-gutter-width.

There seems to be no need to set the width of #content when no sidebars are displayed.

Fix is to remove the following code from fixed-width.sccs:

/*
 * The layout when there are no sidebars.
 */

#content {
  width: $zen-grid-width;
}