Remove ReactTestUtils from ReactLegacyUpdates-test

[eps1lon]

[react-sizebot]

Comparing: 850fac4915864a487e7cb9ecae8a75dbac144174...41b703247550069e6a53ab9e68bc4de0296e484f

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	=	177.10 kB	177.10 kB	=	55.20 kB	55.20 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	177.64 kB	177.64 kB	=	55.53 kB	55.53 kB
facebook-www/ReactDOM-prod.classic.js	=	594.34 kB	594.34 kB	=	104.96 kB	104.96 kB
facebook-www/ReactDOM-prod.modern.js	=	577.60 kB	577.60 kB	=	102.02 kB	102.02 kB
test_utils/ReactAllWarnings.js	Deleted	66.60 kB	0.00 kB	Deleted	16.28 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
test_utils/ReactAllWarnings.js	Deleted	66.60 kB	0.00 kB	Deleted	16.28 kB	0.00 kB

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