Remove allowConcurrentByDefault flag (#30445)

Following https://github.com/facebook/react/pull/30436

Concurrent by default strategy has been unshipped. Here we clean up the
`allowConcurrentByDefault` path and related logic/tests.

For now, this keeps the `concurrentUpdatesByDefaultOverride` argument in
`createContainer` and `createHydrationContainer` and ignores the value
to prevent more breaking changes to `react-reconciler` in the RC stage.