Remove unnecessary type cast

[eps1lon]

Summary

Increases type coverage from 82.50% to 82.66% 💪 ...

... just because we got rid of one expression.

$ git checkout master
$ npx flow coverage packages/react-dom/src/client/ReactDOMLegacy.js
Covered: 82.50% (429 of 520 expressions)

$ git checkout eps1lon/chore/root-coverage
$ npx flow coverage packages/react-dom/src/client/ReactDOMLegacy.js 
Covered: 82.66% (429 of 519 expressions)

TODO comment was added in 1b55ad2. The flow bug was likely fixed during an update to flow since the type didn't change in the meantime.

Test Plan

• CI green

[sizebot]

Comparing: 9a25916...1b4bb4f

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	=	122.76 kB	122.76 kB	=	39.42 kB	39.42 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	129.34 kB	129.34 kB	=	41.51 kB	41.51 kB
facebook-www/ReactDOM-prod.classic.js	=	406.92 kB	406.92 kB	=	75.31 kB	75.31 kB
facebook-www/ReactDOM-prod.modern.js	=	394.95 kB	394.95 kB	=	73.41 kB	73.41 kB
facebook-www/ReactDOMForked-prod.classic.js	=	406.92 kB	406.92 kB	=	75.31 kB	75.32 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 1b4bb4f

[bvaughn]

Thanks!