MDL-80730 theme: Remove unused SCSS files

The following files in theme/boost/scss/moodle  were not imported
so they can be removed because the styles defined in them are not
used anywhere:
- editor.scss
- expendable.scss

theme/boost/scss/moodle/editor.scss

@@ -1,5 +0,0 @@
-// Over ride bootstrap editor CSS.
-
-body {
-    margin: 8px;
-}

theme/boost/scss/moodle/expendable.scss

@@ -1,43 +0,0 @@
-table.flexible,
-.generaltable {
-    tbody tr:nth-of-type(#{$table-striped-order}) {
-        background-color: $table-accent-bg;
-    }
-}
-
-table {
-    caption {
-        font-size: 24px;
-        font-weight: bold;
-        line-height: 42px;
-        text-align: left;
-        caption-side: top;
-    }
-}
-
-#page-report-loglive-index .generaltable,
-#page-admin-report-log-index .generaltable,
-#page-report-log-user .generaltable,
-#page-admin-user table,
-.environmenttable,
-.category_subcategories,
-.rcs-results {
-    th,
-    td {
-        padding: $table-cell-padding-sm;
-    }
-}
-
-.forumheaderlist,
-.generaltable,
-table.flexible,
-.category_subcategories,
-table#modules,
-table#permissions {
-    tbody tr {
-        @include hover {
-            color: $table-hover-color;
-            background-color: $table-hover-bg;
-        }
-    }
-}