Remove v19 beta specific issue template

[jackpope]

This was a template for the 19 beta. Since 19 has been stable for a while now, we can clean this up. Any bug report for React 19 should use the standard bug report template.

[react-sizebot]

Comparing: 95671b4...ee2db6e

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	=	6.68 kB	6.68 kB	=	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	515.13 kB	515.13 kB	=	91.78 kB	91.78 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	615.28 kB	615.28 kB	=	108.88 kB	108.88 kB
facebook-www/ReactDOM-prod.classic.js	=	647.74 kB	647.74 kB	=	114.47 kB	114.47 kB
facebook-www/ReactDOM-prod.modern.js	=	638.02 kB	638.02 kB	=	112.89 kB	112.89 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

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