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, 0 insertions, 4 deletions

diff --git a/docs/layout/media-object.md b/docs/layout/media-object.md
index 846330604..a42cda40f 100644
--- a/docs/layout/media-object.md
+++ b/docs/layout/media-object.md
@@ -7,10 +7,6 @@ group: layout
 
 The [media object](http://www.stubbornella.org/content/2010/06/25/the-media-object-saves-hundreds-of-lines-of-code/) is an abstract element used as the basis for building more complex and repetitive components (like blog comments, Tweets, etc). Included is support for left and right aligned content, content alignment options, nesting, and more.
 
-{% callout info %}
-**Heads up!** When [flexbox mode]({{ site.baseurl }}/getting-started/flexbox/) is enabled, the media object will use `flex` styles wherever possible.
-{% endcallout %}
-
 ## Contents
 
 * Will be replaced with the ToC, excluding the "Contents" header