Remove `use_frameworks` hack from Podspec

[kraenhansen]

We're relying on a dirty hack to detect the use of the use_framework directive in the consuming applications Podfile.

https://github.com/realm/realm-js/blob/f84f3321764831b096848002c7c2bfbffe30f3e6/packages/realm/RealmJS.podspec#L5-L25

With React Native 0.71.0 a USE_FRAMEWORKS environment variable is set if the consuming app is configured to use frameworks. This is the commit bringing in the change: https://github.com/facebook/react-native/commit/f7b35c0d434a5e98228311722926819ffe80c4b6

We could either perform the same check (if we don't want to bump our minimum RN version to 0.71.0 - released Jan 12, 2023) or simply rely on the value of the USE_FRAMEWORKS environment variable.