openssl: Don't unload providers

There is a conflict between atexit() handlers registered by OpenSSL and some executables (e.g. swanctl or pki) to deinitialize libstrongswan. Because plugins are usually loaded after atexit() has been called, the handler registered by OpenSSL will run before our handler. So when the latter destroys the plugins it's a bad idea to try to access any OpenSSL objects as they might already be invalid.