Remove feature flag reference from docs

doc/user/markdown.md

@@ -444,7 +444,6 @@ GFM recognizes the following:
 | snippet                         | `$123`                     | `namespace/project$123`                 | `project$123`                  |
 | epic **(ULTIMATE)**             | `&123`                     | `group1/subgroup&123`                   |                                |
 | vulnerability **(ULTIMATE)** (1)| `[vulnerability:123]`      | `[vulnerability:namespace/project/123]` | `[vulnerability:project/123]`  |
-| feature flag                    | `[feature_flag:123]`       | `[feature_flag:namespace/project/123]`  | `[feature_flag:project/123]`   |
 | label by ID                     | `~123`                     | `namespace/project~123`                 | `project~123`                  |
 | one-word label by name          | `~bug`                     | `namespace/project~bug`                 | `project~bug`                  |
 | multi-word label by name        | `~"feature request"`       | `namespace/project~"feature request"`   | `project~"feature request"`    |