Remove IE9 browser hacks. (#21393)

docs/components/buttons.md

@@ -105,10 +105,6 @@ Buttons will appear pressed (with a darker background, darker border, and inset
 
 Make buttons look inactive by adding the `disabled` boolean attribute to any `<button>` element.
 
-{% callout info %}
-**Heads up!** IE9 and below render disabled buttons with gray, shadowed text that we can't override.
-{% endcallout %}
-
 {% example html %}
 <button type="button" class="btn btn-lg btn-primary" disabled>Primary button</button>
 <button type="button" class="btn btn-secondary btn-lg" disabled>Button</button>

docs/components/forms.md

@@ -933,8 +933,6 @@ Custom `<select>` menus need only a custom class, `.custom-select` to trigger th
 </select>
 {% endexample %}
 
-Custom selects degrade nicely in IE9, receiving only a handful of overrides to remove the custom `background-image`. **Multiple selects (e.g., `<select multiple>`) are not currently supported.**
-
 ### File browser
 
 The file input is the most gnarly of the bunch and require additional JavaScript if you'd like to hook them up with functional *Choose file...* and selected file name text.