RingRTC (along with WebRTC) are no longer included as submodules; now
they are a nearly-standard pod dependency with an extra "prebuild
checksum" to download the built artifacts.
This removes the submodules as well as the storing of the WebRTC
artifact repo commit in the app's Info.plist. RingRTC already prints
the current version in debug logs, so we're not losing anything.
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