Flexbox all the time (Drop IE9 support and remove $enable-flex option) (#21389)

* remove the $enable-flex variable option

* remove bootstrap-flex.css dist file and it's grunt task

* remove the separate flex css file for docs; it's all the same now

* remove flexbox docs (porting some to the main grid docs in next commit)

* clean up few grid docs bits to simplify copy, start to mention flexbox

* port relevant flexbox-grid.md content to grid.md

- clean up mixins
- update how it works section
- bring over sizing and alignment sections

* remove the $enable-flex from the options.md page

* update lead paragraph to mention flexbox

* update migration to mention loss of ie9 support

* remove mention of flexbox dist file

* clarify IE support

* making a note

* remove flexbox variant mentions from component docs

- updates docs for media object, navs, list group, and cards to consolidate docs
- no more need to callout flexbox variants since it's now the default

* remove $enable-flex if/else from sass files

* remove flex dist files

* update scss lint property order to account for flex properties

* linting

* change to numberless classes for autosizing, wrap in highlighting div

* bump gruntfile and postcss to ie10

* redo intro sections

* rearrange

* phew, redo hella grid docs

- rearrange all the things
- consolidate some bits

* remove reference to flexbox mode

* more border action for demo

* Make some changes to the .card's in .card-deck's to ensure footers align to the bottom

1 files changed, 1 insertions, 20 deletions

diff --git a/docs/components/list-group.md b/docs/components/list-group.md
index 8a02f9864..1c1384724 100644
--- a/docs/components/list-group.md
+++ b/docs/components/list-group.md
@@ -27,26 +27,7 @@ The most basic list group is simply an unordered list with list items, and the p
 
 ## Badge
 
-Add badges to any list group item to show unread counts, activity, and more with the help of some utilities. Below we use `.float-right` and `.mt-1` to align the badge to this example's specific needs, but you may need different or additional utilities.
-
-{% example html %}
-<ul class="list-group">
-  <li class="list-group-item">
-    <span class="badge badge-default badge-pill float-right mt-1">14</span>
-    Cras justo odio
-  </li>
-  <li class="list-group-item">
-    <span class="badge badge-default badge-pill float-right mt-1">2</span>
-    Dapibus ac facilisis in
-  </li>
-  <li class="list-group-item">
-    <span class="badge badge-default badge-pill float-right mt-1">1</span>
-    Morbi leo risus
-  </li>
-</ul>
-{% endexample %}
-
-When in flexbox mode, you'll need to rearrange the contents of your list group items. To replicate the right-aligned badges as shown above, use the following example's code. Note the [`flex-items-between` utility class]({{ site.baseurl }}/layout/flexbox-grid/#horizontal-alignment), the badge's placement, and the lack of a float and margin utilities on the badges.
+Add badges to any list group item to show unread counts, activity, and more with the help of some utilities. Note the [`flex-items-between` utility class]({{ site.baseurl }}/layout/grid/#horizontal-alignment), the badge's placement, and the lack of a float and margin utilities on the badges.
 
 {% highlight html %}
 <ul class="list-group">