This option was previously only in tests to avoid
emqx_conf app overwriting previously set configs with default values.
After a03f2dd64b, the issue for
test cases had been resolved.
This commit is to get rid of the option all together
emqx_config_handler:post_config_update/5 cb is called before an updated config is saved.
Thus, a process being restarted in that callback cannot get the latest config by calling
emqx_conf:get/2, because that update is not saved yet.
Relates to EMQX-9055
Zaiming (Stone) Shi