nm-settings-connection: Remove the system secrets cache

As analyzed in #599 (closed) NMSettingsConnection's priv->system_secrets and the dance that's done to copy the secrets to and from it seems to be a left-over and not currently needed. After looking at all users of nm_settings_connection_clear_secrets I conclude that nothing depends on it so I'm dropping that function and the system secrets cache as proposed in #599 (closed) and simplifying some code paths. My testing was limited to running the unit tests and checking a few Wi-Fi scenarios with wrong secrets and correct secrets provided through the agent UI with the IWD and wpa_supplicant backends.

This MR is based on !813 (closed) as that ones is less invasive. There are only two new commits on top of !813 (closed).

Note: there's also an agent-owned secrets cache in NMSettingsConnection that is only used once, to preserve the agent-owned secrets over a D-Bus .Update() call.