Don't show already Linked Facilities on Link New Facility on User page

[Pranshu1902]

Describe the bug
When a user tries to link a facility to the user, the already linked facilities are also shown.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://care.coronasafe.in/users
2. Click on Linked Facility
3. In the form, add a facility that is already linked and click Add
4. See error

Expected behavior
Only those facilities that are not linked to the user should be displayed.

Screenshots

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.

[Pranshu1902]

Issue raised in Backend