Change Runner back to shared after marking it as specific
### Summary In the admin area where you setup your shared Runners, you have the option to assign them to specific projects and then they become specific. Once you do that, you can no longer turn it back to shared. ### Steps to reproduce 1. Go to `/admin/runners` 1. Register a new Runner and watch its type being `specific` 1. Edit that Runner and select a project from the **Restrict projects for this Runner** list 1. Disable that project from the list 1. The Runner is still specific Tested on latest master.
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