[REM] extensions: remove the 'switcher' extension

The 'sphinx-tabs' extension is better in every aspect:
- Synchronize selected tabs (group tabs feature).
- No need for extra `code-block` directive (code tabs feature).
- Better looking.
- No custom Python, JS, or CSS.

task-2787415

Part-of: odoo/documentation#1688

extensions/odoo_theme/static/style.scss

@@ -512,25 +512,6 @@ header.o_main_header {
                   max-width: 100%;
                 }
 
-                .content-switcher {
-                    .nav-tabs .nav-link.active {
-                        background-color: #f8f8f8;
-                        border-color: #dee2e6 #dee2e6 #f8f8f8;
-                        font-weight: $fw_semibold;
-                    }
-
-                    .tab-content {
-                        background: $doc_code-bg;
-                        border: 1px $gray-300 solid;
-                        border-top: 0;
-
-                        div[class^="highlight"] {
-                            border: 0;
-                            margin: 0;
-                        }
-                    }
-                }
-
                 @include media-breakpoint-up(xl) {
                     width: 100%;
                     padding-right: 0;