Fix: sessiond: error accounting: unchecked lttng_ht_del return value

There is no reason for this deletion to fail; assert that it succeeds
to detect any internal error.

1451556 Unchecked return value

If the function returns an error value, the error value may be mistaken for a normal value.

In event_notifier_error_accounting_unregister_event_notifier: Value returned from a function is not checked for errors before being used (CWE-252)

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I3d0f8995608ac96cd3c94b5e92c4459fff592702