Refactor system setting (#27000)

This PR reduces the complexity of the system setting system.

It only needs one line to introduce a new option, and the option can be
used anywhere out-of-box.

It is still high-performant (and more performant) because the config
values are cached in the config system.

options/locale/locale_en-US.ini

@@ -3157,7 +3157,6 @@ config.access_log_mode = Access Log Mode
 config.access_log_template = Access Log Template
 config.xorm_log_sql = Log SQL
 
-config.get_setting_failed = Get setting %s failed
 config.set_setting_failed = Set setting %s failed
 
 monitor.stats = Stats