Remove ReadLineSafe from known issues

docs/KNOWNISSUES.md

@@ -1,18 +1,5 @@
 # Known Issues
 
-## SecureString
-
-The `SecureString` class is *not* the well-known C# `SecureString`, as the
-library is not available in .NET Core. However, PowerShell continues to use the
-type for two main reasons: FullCLR compatibility, and as a specially-treated
-type (that is, behavior is dependent on the type being `SecureString`). Instead,
-the `SecureString` is Mono's completely not secure whatsoever, but open-source
-and "compatible" stub that acts just like a plaintext `StringBuilder`. **It has
-no encryption.**
-
-Additionally `ReadLineSafe` is not implemented, meaning `Get-Credential` fails
-with `PlatformNotSupportedException`.
-
 ## Files excluded from the build
 
 #### Microsoft.PowerShell.Commands.Management