Remove use of react-reconciler/reflection

[aweary]

See #1399 (comment) for details. This seemed like a good idea at the time, but it will probably cause headaches in the future. We need to either just revert #1399 or think of a better solution. Related to #1538, maybe we could have react-is/reflection?

Either way, we should probably undo this.

[ljharb]

I’m fine reverting, but would it be better to expose an actual synced package for this?

[ljharb]

This is done for the 16 adapter, and future adapters, in #1778 / #1790.