feat: remove deprecated Alert shortcode which was replaced by Callout

wowchemy/layouts/shortcodes/alert.html

@@ -1,7 +0,0 @@
-{{/* Todo: to be deprecated in favour of Callout. */}}
-{{ warnf "Alert shortcode will be deprecated in future. Use Callout instead. Rename `alert` to `callout` in %q" $.Page.Path }}
-<div class="alert alert-{{ .Get 0 }}">
-  <div>
-    {{ .Inner | markdownify | emojify }}
-  </div>
-</div>