Remove some CSS extends

Group page dropdown was recently replaced with a gl- one
Editor page only needs font family
The .key classes didn't make the move to shortcut_mappings.vue

app/assets/stylesheets/pages/commits.scss

-%commit-description-base {
+.commit-description,
+.commit-row-description {
   padding: $gl-padding-8 0 $gl-padding-8 $gl-padding-8;
   margin-top: $gl-padding-8;
   border: 0;
@@ -10,10 +11,6 @@
   color: $gl-text-color-secondary;
 }
 
-.commit-description {
-  @extend %commit-description-base;
-}
-
 .commit-box {
   border-top: 1px solid $border-color;
   padding: $gl-padding 0;
@@ -249,7 +246,6 @@
   }
 
   .commit-row-description {
-    @extend %commit-description-base;
     display: none;
     flex: 1;
   }

app/assets/stylesheets/pages/editor.scss

@@ -36,7 +36,7 @@
   }
 
   .file-title {
-    @extend .monospace;
+    @include gl-font-monospace;
     line-height: 35px;
     padding-top: 7px;
     padding-bottom: 7px;

app/assets/stylesheets/pages/groups.scss

@@ -160,17 +160,6 @@
     vertical-align: top;
   }
 
-
-  .notification-dropdown {
-    .dropdown-menu {
-      @extend .dropdown-menu-right;
-    }
-
-    .icon {
-      fill: $gl-text-color-secondary;
-    }
-  }
-
   .new-project-subgroup {
     .dropdown-primary {
       min-width: 115px;

app/assets/stylesheets/pages/help.scss

@@ -26,13 +26,6 @@
     text-align: right;
     white-space: nowrap;
   }
-
-  .key {
-    @extend .badge.badge-pill;
-    background-color: $label-inverse-bg;
-    font: 11px Consolas, 'Liberation Mono', Menlo, Courier, monospace;
-    padding: 3px 5px;
-  }
 }
 
 .documentation {

app/assets/stylesheets/pages/issuable.scss

@@ -633,7 +633,7 @@
   }
 
   .btn-link:hover {
-    @extend a:hover;
+    color: $blue-800;
     text-decoration: none;
   }
 

app/assets/stylesheets/pages/projects.scss

@@ -141,10 +141,6 @@
     }
   }
 
-  .notification-dropdown .dropdown-menu {
-    @extend .dropdown-menu-right;
-  }
-
   .download-button {
     @include media-breakpoint-down(md) {
       margin-left: 0;
@@ -843,7 +839,7 @@ pre.light-well {
   }
 
   .form-control {
-    @extend .monospace;
+    @include gl-font-monospace;
     background-color: $white;
     border-color: $border-color;
     font-size: 14px;