This assert could be triggered if ALTER EVENT failed to load the
event after altering it. Failing to load the event could for 
example happen because of KILL QUERY.

The assert tested that the result of a failed load_named_event()
was OP_LOAD_ERROR. However since load_named_event() returns bool,
this assert did not make any sense. This patch therefore removes
the assert, fixing the problem. The patch also removes 
enum_events_error_code since it was unused.

No test case added. The bug fix is trivial and this bug was
easily detected by RQG tests. Further, adding a MTR test case
for this bug would require adding sync points to make the
test case repeatable.