Revert "Emit reactroot attribute on the first element we discover" #21340
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This reverts #21154.
The idea was to implement data-reactroot for parity with the old renderer so that we could run the tests without manually branching on hundreds of tests. However, this fix wasn't quite equivalent because the original root wasn't well defined. It was only injected when it was the first element and not when abstractions are used. At least for some of them. I'm not actually even sure what the semantics are.
It actually started failing more tests than it helped with.
This also broke something internally that relied on it not being added.
So trying to fix this just made it worse.