d/r/development: remove docs on experimental persistent per user mount namespace feature

docs/reference/development/experimental-features.md

@@ -22,7 +22,6 @@ The following table lists all the features considered experimental.
 | {ref}`Hotplug support <interfaces-hotplug-support>`                                                       | Jul 2018 (snapd 2.34)   | NO                 | Currently only serial ports are supported (serial-port interface).                           |
 | {ref}`Parallel installs <interfaces-parallel-installs>`                                                      | Jun 2018 (snapd 2.34)   | NO                 | Layouts are not working properly with parallel installs due problems with persistent mounts. |
 | [snapd-snap](https://github.com/canonical/snapd/pull/6404)                        | Oct 2018 (snapd 2.36)   | NO                 | Used for triggering the snapd snap install change.                                           |
-| [per-user-mount-namespace](https://github.com/canonical/snapd/pull/6149)          | Jan 2019 (snapd 2.37)   | NO                 |                                                                                              |
 | {ref}`Refresh app awareness <explanation-how-snaps-work-refresh-awareness>`                                                 | Mar 2019 (snapd 2.38)   | YES                | There are some bugs that need to be addressed.                                               |
 | [robust-mount-namespace-updates](https://github.com/canonical/snapd/pull/7773)     | Nov 2019 (snapd 2.42.3) | YES                | No longer considered experimental.                                                           |
 | [classic-preserves-xdg-runtime-dir](https://github.com/canonical/snapd/pull/7659) | Jan 2020 (snapd 2.43)   | YES                | No longer considered experimental.                                                           |