Stop nulling out fields in SafeDeleteSslContext dispose. (#28889)

Nulling out fields in SafeDeleteSslContext.Dispose causes occasional difficult to reproduce exceptions in CI. The fix is to stop nulling out these fields in the dispose method, and to allow the garbage collector to take care of them instead.

Fixes: #28759

Author: rmkerr

src/System.Net.Security/src/System/Net/Security/Pal.OSX/SafeDeleteSslContext.cs

@@ -114,11 +114,6 @@ protected override void Dispose(bool disposing)
                     _sslContext.Dispose();
                     _sslContext = null;
                 }
-
-                _toConnection = null;
-                _fromConnection = null;
-                _writeCallback = null;
-                _readCallback = null;
             }
 
             base.Dispose(disposing);