Remove unstable_concurrentUpdateByDefault option from RTR

[jackpope]

Clean up unused ReactTestRenderer option: unstable_concurrentUpdateByDefault

[react-sizebot]

Comparing: 9ffe910...be25aae

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	176.83 kB	176.83 kB	=	54.91 kB	54.91 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	173.55 kB	173.55 kB	=	54.11 kB	54.11 kB
facebook-www/ReactDOM-prod.classic.js	=	594.04 kB	594.04 kB	=	104.37 kB	104.37 kB
facebook-www/ReactDOM-prod.modern.js	=	577.30 kB	577.30 kB	=	101.42 kB	101.42 kB
test_utils/ReactAllWarnings.js	Deleted	66.87 kB	0.00 kB	Deleted	16.36 kB	0.00 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
facebook-react-native/react-test-renderer/cjs/ReactTestRenderer-profiling.js	=	331.04 kB	330.34 kB	=	58.14 kB	58.01 kB
facebook-react-native/react-test-renderer/cjs/ReactTestRenderer-prod.js	=	315.14 kB	314.43 kB	=	55.67 kB	55.55 kB
test_utils/ReactAllWarnings.js	Deleted	66.87 kB	0.00 kB	Deleted	16.36 kB	0.00 kB

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