With 3153 we aim to unify the different build systems in realm-js around CMake. However, we still need to maintain some Xcode and Cocoapods projects files for ReactNative iOS consumers.

I propose that we refactor the iOS build files so that they no longer build the SDK from source. Instead, we should use CMake to produce a static library for iOS, and pare down the Xcode and Cocoapods project builds so that they only know how to link against this prebuilt static library, just like we do for ReactNative Android with the prebuilt .so.