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

[effectfully]

In PlutusIR.Compiler.Types we have

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.