Fabric: Removing `Scheduler::shadowTreeRegistry_`
Fabric: Removing `Scheduler::shadowTreeRegistry_`

Summary:
It's dead, unused variable.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: sammy-SC

Differential Revision: D18285498

fbshipit-source-id: ee06950b21053ed488d5e64db28b4b0b03fb4471

javache/react-native,myntra/react-native,exponent/react-native,facebook/react-native,hoangpham95/react-native,exponentjs/react-native,facebook/react-native,janicduplessis/react-native,javache/react-native,pandiaraj44/react-native,pandiaraj44/react-native,javache/react-native,myntra/react-native,exponent/react-native,exponentjs/react-native,javache/react-native,pandiaraj44/react-native,arthuralee/react-native,hammerandchisel/react-native,hoangpham95/react-native,myntra/react-native,exponent/react-native,janicduplessis/react-native,facebook/react-native,exponent/react-native,hoangpham95/react-native,pandiaraj44/react-native,janicduplessis/react-native,janicduplessis/react-native,exponentjs/react-native,pandiaraj44/react-native,facebook/react-native,hoangpham95/react-native,hammerandchisel/react-native,myntra/react-native,myntra/react-native,hammerandchisel/react-native,exponentjs/react-native,myntra/react-native,myntra/react-native,arthuralee/react-native,exponent/react-native,exponentjs/react-native,hammerandchisel/react-native,myntra/react-native,myntra/react-native,arthuralee/react-native,hoangpham95/react-native,pandiaraj44/react-native,javache/react-native,arthuralee/react-native,facebook/react-native,javache/react-native,javache/react-native,exponent/react-native,pandiaraj44/react-native,hammerandchisel/react-native,janicduplessis/react-native,pandiaraj44/react-native,exponentjs/react-native,facebook/react-native,arthuralee/react-native,janicduplessis/react-native,exponent/react-native,hammerandchisel/react-native,javache/react-native,javache/react-native,hoangpham95/react-native,exponent/react-native,facebook/react-native,hoangpham95/react-native,hoangpham95/react-native,exponentjs/react-native,hammerandchisel/react-native,janicduplessis/react-native,janicduplessis/react-native,exponentjs/react-native,hammerandchisel/react-native,facebook/react-native,facebook/react-native