Remove experimental scheduler flags

[gaearon]

Removes requestIdleCallbackBeforeFirstFrame and requestTimerEventBeforeFirstFrame.

Both were rAF-specific experiments but we went with MessageLoop instead. We left rAF temporarily in the code but since we never actually used these code paths, I think we should delete them to make the code easier to understand.

[acdlite]

Since you've removed this timeout, there's now no timeout fallback for the initial rAF call, like there is for subsequent rAFs:

react/packages/scheduler/src/forks/SchedulerHostConfig.default.js

Lines 306 to 311 in ff00645

	const onTimeout = () => {
	frameDeadline = getCurrentTime() + frameLength / 2;
	performWorkUntilDeadline();
	rAFTimeoutID = setTimeout(onTimeout, frameLength * 3);
	};
	rAFTimeoutID = setTimeout(onTimeout, frameLength * 3);

But it's whatever since we're going to disable the rAF stuff anyway.

[acdlite]

That timeout should have already been in the else branch of this feature, anyway.

[gaearon]

there's now no timeout fallback for the initial rAF call, like there is for subsequent rAFs

I guess that explains #16629?

[sizebot]

React: size: 0.0%, gzip: 0.0%

Details of bundled changes.

Comparing: 040ca0f...51f728a

react

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
react.development.js	-1.7%	-1.5%	119.56 KB	117.47 KB	30.27 KB	29.82 KB	UMD_DEV
react.production.min.js	0.0%	0.0%	12.43 KB	12.43 KB	4.9 KB	4.9 KB	UMD_PROD
react.profiling.min.js	0.0%	0.0%	15.83 KB	15.83 KB	5.9 KB	5.9 KB	UMD_PROFILING
react.development.js	0.0%	0.0%	73.12 KB	73.12 KB	19.25 KB	19.25 KB	NODE_DEV
react.production.min.js	0.0%	🔺+0.1%	6.69 KB	6.69 KB	2.78 KB	2.78 KB	NODE_PROD
React-dev.js	0.0%	0.0%	71.18 KB	71.18 KB	18.31 KB	18.31 KB	FB_WWW_DEV
React-prod.js	0.0%	0.0%	17.41 KB	17.41 KB	4.55 KB	4.56 KB	FB_WWW_PROD
React-profiling.js	0.0%	0.0%	17.41 KB	17.41 KB	4.55 KB	4.56 KB	FB_WWW_PROFILING

scheduler

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
scheduler-unstable_mock.development.js	-0.0%	0.0%	22.12 KB	22.12 KB	5.13 KB	5.13 KB	UMD_DEV
scheduler-tracing.profiling.min.js	0.0%	+0.3%	3.25 KB	3.25 KB	990 B	993 B	NODE_PROFILING
scheduler-unstable_mock.production.min.js	0.0%	0.0%	4.73 KB	4.73 KB	1.98 KB	1.98 KB	UMD_PROD
Scheduler-dev.js	-6.8%	-5.9%	31.76 KB	29.62 KB	7.99 KB	7.52 KB	FB_WWW_DEV
scheduler-tracing.development.js	0.0%	+0.1%	11.72 KB	11.72 KB	3.03 KB	3.03 KB	NODE_DEV
scheduler-tracing.production.min.js	0.0%	🔺+0.8%	728 B	728 B	380 B	383 B	NODE_PROD
scheduler-unstable_mock.development.js	-0.0%	0.0%	21.93 KB	21.93 KB	5.07 KB	5.07 KB	NODE_DEV
scheduler-unstable_mock.production.min.js	0.0%	🔺+0.1%	4.72 KB	4.72 KB	1.92 KB	1.92 KB	NODE_PROD
scheduler.development.js	-6.7%	-5.7%	31.26 KB	29.16 KB	7.88 KB	7.43 KB	NODE_DEV

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