[refactor] remove unused fiberstack functions

[rickhanlonii]

These haven't been used since we landed the new scheduler.

Seems like maybe they should be? But if not, let's delete them.

[react-sizebot]

Comparing: fa3feba...2cd8c6c

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	+0.05%	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	530.57 kB	530.57 kB	=	93.67 kB	93.67 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	+0.05%	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	651.66 kB	651.66 kB	=	114.78 kB	114.78 kB
facebook-www/ReactDOM-prod.classic.js	=	674.81 kB	674.81 kB	=	118.78 kB	118.78 kB
facebook-www/ReactDOM-prod.modern.js	=	665.30 kB	665.30 kB	=	117.19 kB	117.19 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 2cd8c6c