Remove builder API (e.g., `b.getFloat4E2M1FNType()`) and caching in `MLIRContext` for low-precision FP types. Types are still cached in the type uniquer. For details, see: https://discourse.llvm.org/t/rethink-on-approach-to-low-precision-fp-types/82361/28 Note for LLVM integration: Use `b.getType<Float4E2M1FNType>()` or `Float4E2M1FNType::get(b.getContext())` instead of `b.getFloat4E2M1FNType()`.