Demos: Remove references to iconshadow

[gabrielschulhof]

$ git grep -nH iconshadow | awk -F ':' '{ print $1; }' | sort -u
demos/icons-grunticon/index.php
demos/listview-grid/index.php
demos/listview-grid/listview-grid.html
demos/old-faq-pages/why-wont-my-css-styles-apply-correctly-to-a-widget.php
demos/panel-swipe-open/index.php
demos/theme-classic/index.php

[jaspermdegroot]

I included this in the demos To Do list at #8245 and am closing this ticket as duplicate.