[Native] Enable and remove targetAsInstance feature flag.

[elicwhite]

Summary

This has been enabled by default in React Native at Facebook and additional changes have landed that require this to be enabled. Removing the flag.

Test Plan

Flag is already enabled at FB and verified. Jest, for the removal changes

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Latest deployment of this branch, based on commit 633107c:

Sandbox	Source
serene-wu-jexdy	Configuration

[sizebot]

Details of bundled changes.

Comparing: 4469700...633107c

react-native-renderer

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
ReactNativeRenderer-prod.js	0.0%	0.0%	260.62 KB	260.65 KB	45.23 KB	45.23 KB	RN_OSS_PROD
ReactNativeRenderer-profiling.js	0.0%	0.0%	271.77 KB	271.81 KB	47.36 KB	47.36 KB	RN_OSS_PROFILING
ReactFabric-prod.js	0.0%	🔺+0.1%	252.52 KB	252.61 KB	43.74 KB	43.77 KB	RN_OSS_PROD
ReactFabric-profiling.js	0.0%	0.0%	263.68 KB	263.77 KB	45.88 KB	45.9 KB	RN_OSS_PROFILING
ReactFabric-dev.js	-0.1%	-0.1%	616.62 KB	616.05 KB	133.12 KB	132.94 KB	RN_FB_DEV
ReactFabric-prod.js	-0.1%	-0.2%	253.14 KB	252.77 KB	43.88 KB	43.8 KB	RN_FB_PROD
ReactNativeRenderer-dev.js	0.0%	0.0%	631.25 KB	631.32 KB	136.86 KB	136.87 KB	RN_OSS_DEV
ReactFabric-profiling.js	-0.1%	-0.2%	264.29 KB	263.92 KB	46.02 KB	45.94 KB	RN_FB_PROFILING
ReactNativeRenderer-dev.js	-0.1%	-0.1%	634.59 KB	633.95 KB	137.39 KB	137.21 KB	RN_FB_DEV
ReactNativeRenderer-prod.js	-0.2%	-0.2%	261.2 KB	260.79 KB	45.34 KB	45.26 KB	RN_FB_PROD
ReactNativeRenderer-profiling.js	-0.1%	-0.2%	272.35 KB	271.95 KB	47.49 KB	47.41 KB	RN_FB_PROFILING
ReactFabric-dev.js	0.0%	0.0%	613.26 KB	613.42 KB	132.58 KB	132.61 KB	RN_OSS_DEV

Size changes (experimental)

Generated by 🚫 dangerJS against 633107c

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No significant bundle size changes to report.

Size changes (stable)

Generated by 🚫 dangerJS against 633107c