Remove unused/unnecessary SASS vars and mixins

- Remove unnecessary clearfix in annotation-header. We don't use floats
  in the layout of annotation threads or annotations any more.

- Remove unused variables

- Remove an unused mixin in `forms.scss`

src/styles/mixins/forms.scss

@@ -14,15 +14,6 @@
   }
 }
 
-// See http://compass-style.org/reference/compass/utilities/general/clearfix/#mixin-pie-clearfix
-@mixin pie-clearfix {
-  &:after {
-    content: '';
-    display: table;
-    clear: both;
-  }
-}
-
 @mixin focus-outline {
   border-color: var.$color-focus-outline;
   box-shadow: 0px 1px 2px rgba(0, 0, 0, 0.075) inset,
@@ -57,14 +48,6 @@
   }
 }
 
-@mixin btn-hover {
-  box-shadow: 0 1px 0 rgba(0, 0, 0, 0.05);
-  outline: none;
-  color: var.$button-text-color;
-  background: var.$button-background-start;
-  border-color: var.$grey-4;
-}
-
 @mixin primary-action-btn {
   @include focus.outline-on-keyboard-focus;
   color: var.$grey-1;

src/styles/sidebar/components/annotation-header.scss

@@ -2,7 +2,6 @@
 @use "../../variables" as var;
 
 .annotation-header {
-  @include forms.pie-clearfix;
   color: var.$grey-6;
 
   &__row {

src/styles/variables.scss

@@ -34,8 +34,6 @@ $grey-6: #3f3f3f;
 
 $grey-7: #202020;
 
-$black: #000 !default;
-
 // Colors
 
 $color-success: #00a36d;
@@ -47,12 +45,6 @@ $highlight: #58cef4;
 $color-focus-outline: #51a7e8;
 $color-focus-shadow: color.scale($color-focus-outline, $alpha: -50%);
 
-$button-text-color: $grey-mid !default;
-$button-background-start: $white !default;
-$button-background-end: #f0f0f0 !default;
-$button-background-gradient: to bottom, $button-background-start,
-  $button-background-end !default;
-
 $error-color: #f0480c !default;
 $success-color: #1cbd41 !default;
 
@@ -123,8 +115,6 @@ $body1-line-height: 1.4em;
 
 $body2-font-size: 14px;
 
-$title-font-weight: bold;
-
 $normal-font-size: 13px;
 $normal-line-height: 17px;
 
@@ -154,7 +144,6 @@ $highlight-color: rgba(255, 255, 60, 0.4);
 $highlight-color-second: rgba(206, 206, 60, 0.4);
 $highlight-color-focus: rgba(156, 230, 255, 0.5);
 $top-bar-height: 40px;
-$group-list-width: 280px;
 $popup-menu-shadow: 0 1px 1px rgba(0, 0, 0, 0.1);
 
 // Mixins