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, 1 deletions

diff --git a/docs/getting-started/options.md b/docs/getting-started/options.md
index 7fddc9c09..b8dc3b583 100644
--- a/docs/getting-started/options.md
+++ b/docs/getting-started/options.md
@@ -32,7 +32,6 @@ You can find and customize these variables for key global options in our `_varia
 | Variable                    | Values                             | Description                                                                            |
 | --------------------------- | ---------------------------------- | -------------------------------------------------------------------------------------- |
 | `$spacer`                   | `1rem` (default), or any value > 0 | Specifies the default spacer value for our spacer utilities.                           |
-| `$enable-flex`              | `true` or `false` (default)        | Swaps `float` and `display: table` styles for `display: flex`.                         |
 | `$enable-rounded`           | `true` (default) or `false`        | Enables predefined `border-radius` styles on various components.                       |
 | `$enable-shadows`           | `true` or `false` (default)        | Enables predefined `box-shadow` styles on various components.                          |
 | `$enable-gradients`         | `true` or `false` (default)        | Enables predefined gradients via `background-image` styles on various components.      |