Summary: Remove `$Subtype` and `$Supertype` utilities. Replacement was done on a best-effort basis. In many cases it sufficed to replace `$Supertype<T>` and `$Subtype<T>` with `T`, but when this was not possible `any | T` and `any & T` were used instead.
Reviewed By: jbrown215
Differential Revision: D13624765
fbshipit-source-id: 82ee9d43cfc318ed4bd2d84e0f4c5c1d8f488a9c
9b985b1338