Remove some leftover IE9/flex stuff

docs/assets/scss/flex-grid.scss

-// Bootstrap flexbox grid for our docs page
-
-
-//
-// Variables
-//
-
-@import "custom";
-@import "variables";
-
-// Override for flexbox mode
-$enable-flex: true;
-
-//
-// Grid mixins
-//
-
-@import "mixins/clearfix";
-@import "mixins/breakpoints";
-@import "mixins/grid-framework";
-@import "mixins/grid";
-
-//
-// Utilities for flex alignment
-//
-
-@import "utilities/flex";
-
-//
-// Generate the grid class
-//
-
-@import "grid";

scss/_card.scss

@@ -182,14 +182,7 @@
 }
 
 
-// Card set
-//
-// Heads up! We do some funky style resetting here for margins across our two
-// variations (one flex, one table). Individual cards have margin-bottom by
-// default, but they're ignored due to table styles. For a consistent design,
-// we've done the same to the flex variation.
-//
-// Those changes are noted by `// Margin balancing`.
+// Card deck
 
 @include media-breakpoint-up(sm) {
   .card-deck {

scss/bootstrap-grid.scss

 // Bootstrap Grid only
 //
-// Includes relevant variables and mixins for the regular (non-flexbox) grid
-// system, as well as the generated predefined classes (e.g., `.col-4-sm`).
+// Includes relevant variables and mixins for the flexbox grid
+// system, as well as the generated predefined classes (e.g., `.col-sm-4`).
 
 //
 // Box sizing, responsive, and more