Remove deprecated Checkbox component from RNW core.

[chrisglein]

Last documented in 61: https://reactnative.dev/docs/0.61/checkbox
Latest changelog tracking against facebook master has this finally being removed.

[General] [Removed] This diff removes the CheckBox export from React Native. Internally, we are requiring CheckBox directly now and externally people will have to use the community maintained module. (dff17ef by @poteto)

[rectified95]

Splitting this issue in 2 parts - one tracking the reimplementation of the Checkbox module using ABI-safe interfaces, the other to represent us removing the existing implementation from this repo.

Created #5636 to track the implementation work.