dillon1000/react

Clean up enableSyncDefaultUpdates flag a bit (#26858)

## Overview

Does a few things:
- Renames `enableSyncDefaultUpdates` to
`forceConcurrentByDefaultForTesting`
- Changes the way it's used so it's dead-code eliminated separate from
`allowConcurrentByDefault`
- Deletes a bunch of the gated code

The gates that are deleted are unnecessary now. We were keeping them
when we originally thought we would come back to being concurrent by
default. But we've shifted and now sync-by default is the desired
behavior long term, so there's no need to keep all these forked tests
around.

I'll follow up to delete more of the forked behavior if possible.
Ideally we wouldn't need this flag even if we're still using
`allowConcurrentByDefault`.