Make devtools set `spring.messages.reloadable=true`

[candrews]

Devtools should set spring.messages.reloadable=true when it's active, see #13377

[philwebb]

Yeah, I think that would make sense.

[wilkinsona]

With some regret, we've decided to revert this change. The approach taken in #13377 isn't quite right and enabling it by default in DevTools causes some problems that can't be easily addressed in the time available.