Remove `BuiltinCasing` once pattern matching builtins are supported on the chain

In `PlutusIR.Compiler.Types` we have

```haskell
data DatatypeStyle
= ScottEncoding
| SumsOfProducts
| BuiltinCasing
-- ^ A temporary data type style used to make a couple of V3 ledger-api-test tests pass
-- before we can support casing on values of built-in types in newer protocol versions and
-- merge this into 'SumsOfProducts' (which is what controls whether 'Case' is available or
-- not).
```

As the comment says, `BuiltinCasing` is temporary and should be merged into `SumsOfProducts` once pattern matching builtins are enabled on the chain. This issue is for making sure that it does happen.