[System]: Fully stub-out SslStream on platforms where it's not supported (#4802)

[System]: Fully stub-out SslStream on platforms where it's not supported (#4802)

* [System]: Fully stub-out SslStream on platforms where it's not supported.

* Remove 'SECURITY_DEP' conditional from LocalCertificateSelectionCallback.

* Add the internal delegates and remove 'SECURITY_DEP' from SslStream.platformnotsupported.cs.

* In SslStream.cs, we now provide the full API when '!SECURITY_DEP' using the
PlatformNotSupportedException()-based implementation from SslStream.platformnotsupported.cs

* Fix typo.