Remove unused API

[iSazonov]

We need remove unused APIs from the library after merging related PR in PowerShell repo:

• GetUserName Cleanup unused pinvokes on unix PowerShell#9531
• IsSymLink Remove IsSymLink() P/Invoke on Unix PowerShell#9534
• GetFullyQualifiedName Use IPGlobalProperties on all platforms PowerShell#9530

[TravisEz13]

These were removed in 7.0.0-preview.1. So these API's cannot be removed until previous versions are out of support.

[adityapatwardhan]

Cannot remove IsSymLink because FollowSymLink uses it.