Cleanup enableUnifiedSyncLane flag

[tyao1]

Summary

The feature flag enables batching discrete, continuous and default updates. This is being rolled out internally and there hasn't been any issues. The flag will be ready to be removed once our internal roll out is done. Because default and continuous got batched to sync, it has similar behavior as the enableSyncDefaultUpdates flag, and this is now the only behavior in tests. As a result, this PR also cleans up some of enableSyncDefaultUpdates conditions in tests.

How did you test this change?

yarn test

[kassens]

Internal rollout hasn't finished quite yet.

[react-sizebot]

Comparing: da6ba53b10d8240fc251ba14a3e5878604d3dc7d...9e4ee1b89f987a7e17120559f6d0e159929b8bfd

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	=	168.91 kB	168.91 kB	=	52.94 kB	52.94 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	170.57 kB	170.57 kB	=	53.44 kB	53.44 kB
facebook-www/ReactDOM-prod.classic.js	=	590.83 kB	590.60 kB	=	103.91 kB	103.85 kB
facebook-www/ReactDOM-prod.modern.js	=	566.65 kB	566.41 kB	=	100.10 kB	100.04 kB
test_utils/ReactAllWarnings.js	Deleted	64.44 kB	0.00 kB	Deleted	16.10 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	64.44 kB	0.00 kB	Deleted	16.10 kB	0.00 kB

Generated by :no_entry_sign: dangerJS against 9e4ee1b89f987a7e17120559f6d0e159929b8bfd