MDL-66999 theme_boost: @extend replace extends in forms

The css for .form-buttons, .path-admin... has been removed completely
The extend was used to add a grey container which is no longer works

theme/boost/scss/moodle/forms.scss

@@ -123,15 +123,6 @@ input#id_externalurl {
     font-weight: bold;
 }
 
-// Pale grey container for submit buttons.
-.form-buttons,
-.path-admin .buttons,
-.fp-content-center form + div,
-div.backup-section + form {
-    @extend .form-group;
-    padding-left: 0;
-}
-
 .form-item .form-setting .defaultsnext > input {
     display: inline-block;
 }
@@ -471,20 +462,20 @@ textarea[data-auto-rows] {
 .input-group {
     &.form-inset {
         .form-inset-item {
-            @extend .position-absolute;
+            position: absolute;
             padding-top: calc(#{$input-padding-y} + #{$input-border-width});
             z-index: 3;
         }
 
         &.form-inset-left {
             .form-control {
-                @extend .pl-4;
+                padding-left: $spacer * 1.5;
             }
         }
 
         &.form-inset-right {
             .form-control {
-                @extend .pr-4;
+                padding-right: $spacer * 1.5;
             }
             .form-inset-item {
                 right: 0;