Closes https://github.com/zed-industries/zed/issues/42312

The issue here was that we assumed that context servers provided by
extensions would always need a config in the settings to be present when
actually the opposite was the case - context servers provided by
extensions are the only context servers that do not need a config to be
in place in order to be available in the UI.

Release Notes:

- Fixed an issue where context servers provided by extensions could not
be uninstalled if they were previously unconfigured.