Don't show Skills with already linked to the user

[Pranshu1902]

Describe the bug
The Linked Skills drop-down shows the linked skills as well.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://care.coronasafe.in/users
2. Click on Linked Skills
3. Link to a new skill and then try to link it again
4. See error

Expected behavior
Only show those skills that haven't been linked to the user.

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.