Remove internal `act` builds from public modules (#21721)

* Move internal version of act to shared module

No reason to have three different copies of this anymore.

I've left the the renderer-specific `act` entry points because legacy
mode tests need to also be wrapped in `batchedUpdates`. Next, I'll update
the tests to use `batchedUpdates` manually when needed.

* Migrates tests to use internal module directly

Instead of the `unstable_concurrentAct` exports. Now we can drop those
from the public builds.

I put it in the jest-react package since that's where we put our other
testing utilities (like `toFlushAndYield`). Not so much so it can be
consumed publicly (nobody uses that package except us), but so it works
with our build tests.

* Remove unused internal fields

These were used by the old act implementation. No longer needed.