ios: stop using react-native-background-timer
ios: stop using react-native-background-timer

Ever since https://github.com/facebook/react-native/pull/23674 landed it has
been possible to run timers in the background, assuming your app is allowed to
run in the background already, as is our case. So, stop using the library on
iOS, which will avoid creatring needless backgound tasks.

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