openssl: Don't unload providers

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.

Fixes: f556fce16b60 ("openssl: Load "legacy" provider in OpenSSL 3 for algorithms like MD4, DES etc.")
Closes strongswan/strongswan#921