remove unused setters on Flag

The following methods on Flag aren't used in our codebase, or are used only in tests:

- setGlobal($global = TRUE)
- setFlagShortText($text)
- setFlagLongText($flag_long)
- setFlagMessage($flag_message)
- setUnflagLongText($unflag_long)
- setUnflagMessage($unflag_message)
- setUnflagShortText($unflag_short)
- setUnflagDeniedText($unflag_denied_text)
- setWeight($weight)

There's no use case for them, as these are config settings that won't be changed on a Flag entity dynamically. If developers REALLY want to change them, they can use the set() method. We shouldn't support all this code that's useless to us.

Postponed on #2868098: refactor all get(un)Flag*() methods → .