Unrevert "Support writing to this.refs from userspace"

[kassens]

Reverts #28877

We found the cause of the regression and should be able to land this again.

[react-sizebot]

Comparing: 699d03c...e77e16c

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.js	=	497.71 kB	497.69 kB	=	88.93 kB	88.93 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	504.00 kB	503.98 kB	=	89.95 kB	89.94 kB
facebook-www/ReactDOM-prod.classic.js	=	591.14 kB	591.14 kB	=	103.91 kB	103.91 kB
facebook-www/ReactDOM-prod.modern.js	=	566.95 kB	566.95 kB	=	100.12 kB	100.12 kB
test_utils/ReactAllWarnings.js	Deleted	64.80 kB	0.00 kB	Deleted	16.14 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
oss-experimental/react/cjs/react.production.js	=	18.09 kB	18.06 kB	=	4.69 kB	4.68 kB
oss-stable/react/cjs/react.production.js	=	16.97 kB	16.93 kB	=	4.43 kB	4.42 kB
oss-stable-semver/react/cjs/react.production.js	=	16.95 kB	16.91 kB	=	4.41 kB	4.39 kB
test_utils/ReactAllWarnings.js	Deleted	64.80 kB	0.00 kB	Deleted	16.14 kB	0.00 kB

Generated by 🚫 dangerJS against e77e16c

[kassens]

@acdlite I keep the this.refs = {} "unfreezing" the object in mountClassInstance for copied class implementations such as create-create-class, see 4d944b6.
Is there a downside beyond the useless new empty object?