TextInput: Remove legacy iOS implementation
TextInput: Remove legacy iOS implementation

Summary:
This implementation was replaced in January of 2018 by shergin. I believe everyone should have `RCTVirtualText` at this point, which should make this safe to remove.

Changelog:
[Internal][TextInput] Remove deprecated and unused legacyIOS implementation

Reviewed By: shergin

Differential Revision: D18296981

fbshipit-source-id: b5d5756e7bbc8141f1b826ab07c76a781ab03edc

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