Fix documentation for list runners

We are specifying that we will only return the shared runners on two
condition; 1 if project has any available 2 if shared runners are
enabled.

2 is not correct. In the admin UI, when the user press "disable shared
runners" we still show the runners to show what kind of runners are
available when the user enables them. We want the API to mimic this
behavior from the UI.

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52918

Author: steve-mt

doc/api/runners.md

@@ -340,8 +340,7 @@ Example response:
 ## List project's runners
 
 List all runners (specific and shared) available in the project. Shared runners
-are listed if at least one shared runner is defined **and** shared runners
-usage is enabled in the project's settings.
+are listed if at least one shared runner is defined.
 
 ```
 GET /projects/:id/runners