Unfork invariant and instead use it from reactProdInvariant

[gaearon]

Follow-up to #12539. After chatting with @sophiebits we decided it would be clearer if we just used invariant in both places.

Pros:

• Less forks to maintain.
• We have error codes in www (same as Preserve error codes for invariants on www #12539).
• If the error code URL for some reason contains %s, on www we'll no longer attempt to replace it (that was a potential bug in Preserve error codes for invariants on www #12539).

Cons:

• Slightly larger bundles. (A few dozen bytes.)
• In www's invariant implementation, framesToProp might need to account for React errors specifically (and be 2). Or maybe it doesn't matter since we get the exact message through the decoder anyway.

[sophiebits]

cool cool cool