remove unused mixins (#793)

This commit removes css classes generated by UtilityLayouts.scss.
These classes (`l-row-\*`, `l-col-\*`,  `l-centeredColumn`) are not used
anywhere in the theme or SDK. Removing these auto-generated
classes reduces the css bundle size from 267KB to 194KB.

J=SLAP-1298
TEST=manual

loaded up a page, ran a search, asked for a direct answer

static/scss/answers/common/util/UtilityLayout.scss

@@ -1,97 +1,6 @@
 @import "../variables";
 @import "./Spacing.scss";
 
-$generate-classes: false !default;
-$col-gap: spacing(base) !default;
-$splitting-points: (sm, md, lg) !default;
-
-
-@if $generate-classes
-{
-  .l-row
-  {
-    @include l-row;
-
-    &-separator
-    {
-      border-bottom:solid 1px #222;
-    }
-
-    &-vertical-margin
-    {
-      &-top
-      {
-        margin-top: $vertical-margin-top;
-
-        &-sm
-        {
-          margin-top: $vertical-margin-top / 2;
-        }
-      }
-
-      &-bottom
-      {
-        margin-bottom: $vertical-margin-top;
-
-        &-sm
-        {
-          margin-bottom: $vertical-margin-top / 2;
-        }
-      }
-    }
-  }
-
-  @for $colCount from 1 through $grid-columns
-  {
-    @each $start, $end, $direction in (lg, null, "up"),
-                                      (md, null, "up"),
-                                      (sm, null, "up"),
-                                      (null, md, "down"),
-                                      (null, sm, "down"),
-                                      (null, xs, "down")
-    {
-      $abbreviation: if($start, $start, $end);
-
-      .l-col-#{$abbreviation}-#{$colCount}-#{$direction}
-      {
-        @include bp($start, $end)
-        {
-          @include l-col($colCount, $grid-columns)
-        }
-      }
-    }
-  }
-
-  @for $colCount from 1 through $grid-columns
-  {
-    @each $start, $end in (lg, null),
-                          (md, md),
-                          (sm, sm),
-                          (null, xs)
-    {
-      $abbreviation: if($start, $start, $end);
-
-      .l-col-#{$abbreviation}-#{$colCount}
-      {
-        @include bp($start, $end)
-        {
-          @include l-col($colCount, $grid-columns)
-        }
-      }
-    }
-  }
-
-  @each $bp in $splitting-points
-  {
-    @include l-Split($bp, $col-gap, '.l-Split');
-  }
-
-  .l-centeredColumn
-  {
-    @include l-centeredColumn;
-  }
-}
-
 .l-container
 {
   @include l-container;

static/scss/answers/common/variables.scss

@@ -1,6 +1,3 @@
-// utility mixin layout variables
-$generate-classes: true;
-
 $grid-gutter-width: 32px !default;
 $grid-gutter-width-xs: $grid-gutter-width !default;
 $grid-gutter-width-sm: $grid-gutter-width !default;